You are right on the money with your description. I finally broke down and printed the CSS code that's generated and learned a lot about how the extension works. There are a couple of width values set that are not set directly by making changes in the interface, but are referenced to things like the form width setting.
What I learned was that yes, when picking a default theme, it comes with default widths for inputs (listed as small, medium, large, etc.) as well as default text sizes. This can be changed during Edit Design and then when using Edit Form, the input sizes can be selected that may have been modified during the Edit Design phase. The text size set during Edit Design will also have an effect on the ultimate values used and saved in a CSS stylesheet when clicking Finish.
à votre santé!