if (double.TryParse(txtSize.Text, out Diameter)) ; Causes a syntax error. Was this intended to be a one liner if statement? No ; needed if so! Simply add brackets for readability unless it's a single line if statement. With that said! Here is a great starting point that considers what @Jamiec mentions:

namespace WindowsFormsApp3
{
public partial class Form1 : Form
{
    public Form1()
    {
        InitializeComponent();
    }

    private List<Double> ClassAPlus = new List<Double> { 0.080, 0.130, 0.150, 0.200, 0.250, 0.250 };
    private List<Double> ClassAMinus = new List<Double> { 0.030, 0.050, 0.080, 0.100, 0.100, 0.130 };

    private List<Double> ClassBPlus = new List<Double> { 0.080, 0.250, 0.300, 0.400, 0.500, 0.500 };
    private List<Double> ClassBMinus = new List<Double> { 0.030, 0.100, 0.100, 0.150, 0.150, 0.250 };
    private void btnEnter_Click(object sender, EventArgs e)
    {
        double Diameter, Plus, Minus, Nominal, Tolerance;
        bool parseSuccess = double.TryParse(txtSize.Text, out Diameter);
        if (parseSuccess == false)
        {
            return;
        }

        if (rbA.Checked == true && rbMetric.Checked == true)
        {
            if (Diameter <= 1.57)
            {
                Plus = (Diameter + ClassAPlus[0]);
                Minus = (Diameter - ClassAMinus[0]);
                Nominal = (Plus + Minus) / 2;
                Tolerance = Plus - Nominal;
                lblResult.Text = Nominal.ToString("0.000") + "  ±" + Tolerance.ToString("0.000");
                lblBilat.Text = Diameter.ToString("0.000") + "  +" + ClassAPlus[0].ToString("0.000") + " /- " + ClassAMinus[0].ToString("0.000");
                lblRange.Text = (Nominal - Tolerance).ToString("0.000") + " - " + (Nominal + Tolerance).ToString("0.000");
            }
            else if (Diameter >= 1.57 || Diameter <= 6.35)
            {
                Plus = (Diameter + ClassAPlus[1]);
                Minus = (Diameter - ClassAMinus[1]);
                Nominal = (Plus + Minus) / 2;
                Tolerance = Plus - Nominal;
                lblResult.Text = Nominal.ToString("0.000") + "  ±" + Tolerance.ToString("0.000");
                lblBilat.Text = Diameter.ToString("0.000") + "  +" + ClassAPlus[1].ToString("0.000") + " /- " + ClassAMinus[1].ToString("0.000");
                lblRange.Text = (Nominal - Tolerance).ToString("0.000") + " - " + (Nominal + Tolerance).ToString("0.000");
            }
            else if (Diameter >= 6.35 || Diameter <= 12.7)
            {
                Plus = (Diameter + ClassAPlus[2]);
                Minus = (Diameter - ClassAMinus[2]);
                Nominal = (Plus + Minus) / 2;
                Tolerance = Plus - Nominal;
                lblResult.Text = Nominal.ToString("0.000") + "  ±" + Tolerance.ToString("0.000");
                lblBilat.Text = Diameter.ToString("0.000") + "  +" + ClassAPlus[2].ToString("0.000") + " /- " + ClassAMinus[2].ToString("0.000");
                lblRange.Text = (Nominal - Tolerance).ToString("0.000") + " - " + (Nominal + Tolerance).ToString("0.000");
            }
            else if (Diameter >= 12.7 || Diameter <= 19.05)
            {
                Plus = (Diameter + ClassAPlus[3]);
                Minus = (Diameter - ClassAMinus[3]);
                Nominal = (Plus + Minus) / 2;
                Tolerance = Plus - Nominal;
                lblResult.Text = Nominal.ToString("0.000") + "  ±" + Tolerance.ToString("0.000");
                lblBilat.Text = Diameter.ToString("0.000") + "  +" + ClassAPlus[3].ToString("0.000") + " /- " + ClassAMinus[3].ToString("0.000");
                lblRange.Text = (Nominal - Tolerance).ToString("0.000") + " - " + (Nominal + Tolerance).ToString("0.000");
            }
            else if (Diameter > 19.05 || Diameter <= 25.4)
            {
                Plus = (Diameter + ClassAPlus[4]);
                Minus = (Diameter - ClassAMinus[4]);
                Nominal = (Plus + Minus) / 2;
                Tolerance = Plus - Nominal;
                lblResult.Text = Nominal.ToString("0.000") + "  ±" + Tolerance.ToString("0.000");
                lblBilat.Text = Diameter.ToString("0.000") + "  +" + ClassAPlus[4].ToString("0.000") + " /- " + ClassAMinus[4].ToString("0.000");
                lblRange.Text = (Nominal - Tolerance).ToString("0.000") + " - " + (Nominal + Tolerance).ToString("0.000");
            }
            else if (Diameter > 25.4)
            {
                Plus = (Diameter + ClassAPlus[5]);
                Minus = (Diameter - ClassAMinus[5]);
                Nominal = (Plus + Minus) / 2;
                Tolerance = Plus - Nominal;
                lblResult.Text = Nominal.ToString("0.000") + "  ±" + Tolerance.ToString("0.000");
                lblBilat.Text = Diameter.ToString("0.000") + "  +" + ClassAPlus[5].ToString("0.000") + " /- " + ClassAMinus[5].ToString("0.000");
                lblRange.Text = (Nominal - Tolerance).ToString("0.000") + " - " + (Nominal + Tolerance).ToString("0.000");
            }
            else
            {
                if (rbB.Checked == true && rbMetric.Checked == true)
                {
                    if (Diameter <= 1.57)
                    {
                        Plus = (Diameter + ClassBPlus[0]);
                        Minus = (Diameter - ClassBMinus[0]);
                        Nominal = (Plus + Minus) / 2;
                        Tolerance = Plus - Nominal;
                        lblResult.Text = Nominal.ToString("0.000") + "  ±" + Tolerance.ToString("0.000");
                        lblBilat.Text = Diameter.ToString("0.000") + "  +" + ClassBPlus[0].ToString("0.000") + " /- " + ClassBMinus[0].ToString("0.000");
                        lblRange.Text = (Nominal - Tolerance).ToString("0.000") + " - " + (Nominal + Tolerance).ToString("0.000");
                    }
                    else if (Diameter >= 1.57 || Diameter <= 6.35)
                    {
                        Plus = (Diameter + ClassBPlus[1]);
                        Minus = (Diameter - ClassBMinus[1]);
                        Nominal = (Plus + Minus) / 2;
                        Tolerance = Plus - Nominal;
                        lblResult.Text = Nominal.ToString("0.000") + "  ±" + Tolerance.ToString("0.000");
                        lblBilat.Text = Diameter.ToString("0.000") + "  +" + ClassBPlus[1].ToString("0.000") + " /- " + ClassBMinus[1].ToString("0.000");
                        lblRange.Text = (Nominal - Tolerance).ToString("0.000") + " - " + (Nominal + Tolerance).ToString("0.000");
                    }
                    else if (Diameter >= 6.35 || Diameter <= 12.7)
                    {
                        Plus = (Diameter + ClassBPlus[2]);
                        Minus = (Diameter - ClassBMinus[2]);
                        Nominal = (Plus + Minus) / 2;
                        Tolerance = Plus - Nominal;
                        lblResult.Text = Nominal.ToString("0.000") + "  ±" + Tolerance.ToString("0.000");
                        lblBilat.Text = Diameter.ToString("0.000") + "  +" + ClassBPlus[2].ToString("0.000") + " /- " + ClassBMinus[2].ToString("0.000");
                        lblRange.Text = (Nominal - Tolerance).ToString("0.000") + " - " + (Nominal + Tolerance).ToString("0.000");
                    }
                    else if (Diameter >= 12.7 || Diameter <= 19.05)
                    {
                        Plus = (Diameter + ClassBPlus[3]);
                        Minus = (Diameter - ClassBMinus[3]);
                        Nominal = (Plus + Minus) / 2;
                        Tolerance = Plus - Nominal;
                        lblResult.Text = Nominal.ToString("0.000") + "  ±" + Tolerance.ToString("0.000");
                        lblBilat.Text = Diameter.ToString("0.000") + "  +" + ClassBPlus[3].ToString("0.000") + " /- " + ClassBMinus[3].ToString("0.000");
                        lblRange.Text = (Nominal - Tolerance).ToString("0.000") + " - " + (Nominal + Tolerance).ToString("0.000");
                    }
                    else if (Diameter >= 19.05 || Diameter <= 25.4)
                    {
                        Plus = (Diameter + ClassBPlus[4]);
                        Minus = (Diameter - ClassBMinus[4]);
                        Nominal = (Plus + Minus) / 2;
                        Tolerance = Plus - Nominal;
                        lblResult.Text = Nominal.ToString("0.000") + "  ±" + Tolerance.ToString("0.000");
                        lblBilat.Text = Diameter.ToString("0.000") + "  +" + ClassBPlus[4].ToString("0.000") + " /- " + ClassBMinus[4].ToString("0.000");
                        lblRange.Text = (Nominal - Tolerance).ToString("0.000") + " - " + (Nominal + Tolerance).ToString("0.000");
                    }
                    else if (Diameter > 25.4)
                    {
                        Plus = (Diameter + ClassBPlus[5]);
                        Minus = (Diameter - ClassBMinus[5]);
                        Nominal = (Plus + Minus) / 2;
                        Tolerance = Plus - Nominal;
                        lblResult.Text = Nominal.ToString("0.000") + "  ±" + Tolerance.ToString("0.000");
                        lblBilat.Text = Diameter.ToString("0.000") + "  +" + ClassBPlus[5].ToString("0.000") + " /- " + ClassBMinus[5].ToString("0.000");
                        lblRange.Text = (Nominal - Tolerance).ToString("0.000") + " - " + (Nominal + Tolerance).ToString("0.000");
                    }
                }
            }
        }
    }
}
}