Well, TG sent me an email last night saying he'll send me the updated theme, since I'm using the old version apparently (it was on OSWT). We can go from there in terms of seeing what if anything needs to be fixed.
I had tried searching for textarea and cols=100% in all the theme components and never found it. So I wasn't sure if maybe it was in the wp- standard files, since it looks like every single wp site has a textarea that's the same size, and it would be surprising if designers didn't voluntarily choose to do things differently each his own.
The Codex discusses the issue of the textarea being too wide because of problems in the preceding CSS but it doesn't get any more specific about where to look. If any Codex authors are reading this, this would be a nice thing to go into greater detail over. The themes will be put under greater stress as more and more plugins are added (though it seems plugins are not the issue here since I've disabled them all).