My biggest complaint with the sizing of theater view elements is the proportions. The text should not be bold and the thumbnails should be enlarged a bit. It just seems wasteful using bold because thicker letters mean less spacing and more horizontal space taken up, which leads to more line breaks and therefore, even smaller thumbnails. The thumbnails appear to work best with square images but if you have taller thumbnails, like for DVD cover art, then there is a lot of empty space on the sides.
I've been looking through the skin files but haven't really found an easy to change setting anywhere. It is also very personal as far as what settings work best for certain users, screens, resolutions, viewing distances, viewing habits, etc. We may be getting a 1080p projector in the next few months or year and it will be our first true HD anything. We have some nice monitors that exceed HD resolution but we've never had anything that will be used for TV that is basically exactly an HD resolution.
With this, we plan on bringing the total image size up as large as we can. Now, we have around a 110" screen and it's the only way to watch a movie as far as I'm concerned. Now, any 40 - 60 " TV looks tiny and worthless. I like being engulfed by the picture...but my point is that with this new resolution being blown up so large, I'm betting theater view will be a significantly different experience and I will probably change the settings. I may even lower the percentage because everything will be so large.