You can wrap each of your FormChecklistCheckbox with Expanded or Flexible.

The result of this, I tested on a small screen device with large font rendering enabled, is bottom overflow somewhere, which may make widgets overlapping over one other.

To overcome this, I also wrapped the Text and Checkbox widgets used within the FormBuilderField widget with Expanded and the result looked like:

Here, for long text, the bottom appears clipped.

I then also tried wrapping the same Text filed that I previously wrapped with Expanded with FittedBox. Now the text fit, but since we're using Flexible/Expanded and FittedBox, the text appear large somewhere and somewhere they are small. You can try and check for yourself and choose whichever you prefer or just ignore.