So I figure it out, and I'm telling you all this in case anyone saw my post and just didn't reply because they didn't know the answer. Supposedly this doesn't work on all browsers, although it seems to work on the latest Firefox, Chrome, and Opera at least. Instead of doing anything with the <input> tag you have to use the style and set width: 100%, like so
<input type="text" style="width: 100%;">
I believe the same can be done with any other form elements, and I don't see why the same couldn't work if you just made it part of a class with the same width style.
Hope this helps someone