I mentioned what I found in another post regarding the text input widths. It appears as though when using Edit Design, you can set the values for small, medium, large, and X-Large under Form Elements->Textarea (for example) or Form Elements ->Text (and for others such as the combo group, etc.). Once you do this (I didn't like the rather large values for the width of these input fields), you then go to Edit Form, and where you see your textarea or text listed under "Set 1" (for example), you can choose small, medium, large, or X-Large that you set earlier during the Edit Design phase to choose how wide/high the input field will be. It will use whichever value you select and will use the pixel width that you assigned in Edit Design. So that's how it appears you control the width/height of the input fields that will be ulitimately used on your page. Design potential sizes in Edit Design, then choose the designed size of interest in Edit Form.
Again, assign the desired width/height of form inputs using Edit Design that "could" be used during Edit Form, then choose the actual size (small, medium, ...) you want to use during Edit Form (where it shows Line followed by Set x followed by the input in question) to control the width/height of the input field.
Mine too defaults to 15px for form elements by the way. I happen to use ems for my settings (.9ems), so I change the value. It looks as though you can set the font and its size under Form when in Edit Design, but the labels font and point size will override what is set there if you change it (probably because it's more specific if you look at the CSS file's code).
The place where I got into trouble last night was with saving presets while in Edit Form. It seems as though when clicking the category I wanted, it would revert to Login even though I selected something else for the category. I didn't notice it do that and ended up creating a preset that overrode my login preset which was unintentional. Also noticed that choosing a Design could be tricky in a similar way. So you have to be real careful when saving a preset around the name of the Category and Design so that it doesn't overwrite the preset that you didn't intend to alter. It cost me lots of time reconstructing some of the work I overwrote before I realize what was going on.
hope this helps,