To reduce the size of thump use "RoundedSliderThumpShape"

solution resource = https://api.flutter.dev/flutter/material/SliderThemeData/rangeThumbShape.html

SliderTheme(
  data: SliderTheme.of(context).copyWith(
    thumbShape: RoundSliderThumbShape(enabledThumbRadius: 4),
    // to increase or reduce size use
    rangeThumbShape: RoundRangeSliderThumbShape(enabledThumbRadius: 8)
  ),
  child: RangeSlider(
    values: RangeValues(0, 20),
    min: 0,
    max: 100,
    onChanged: (v) {},
  ),
)