A:
You have to edit the Theme style for a CheckBox Node.
You can find it in the Inspector tab (Godot Editor) just select the CheckBox Node and you should find the Theme style section.
If you click on the CheckBox -> Theme -> Rect -> Custom Constants Folder icon you will be able to customize the size of the checkbox:
You can also change the other graphical parts of the checkbox like the selected icon:
Here some resources about editing the theme of nodes:
How do I modify the size of a button?
Changing colors and sizes of Nodes
Hope this helps