How goes it?
With the boxes, I have found that when you are designing the contents of the form, as you know, you can pick input boxes to be - as you state - one of various size.
From my experience with form builder, that then makes the boxes set to the "default" size for whatever theme you are using (blank or otherwise). Then when you edit the form design, you can edit the size for each of those various sizes and choose to save your Form Design edits for later use, as well as, applying to them the current form.
So I believe the idea was that each design had default attributes defined but the extension allowed for complete customization to be fine tuned for whatever the user wants.