By the way, I have some comments with div tags in the beginning, but they don't specify a font size, and those comments look normal-sized on my Nook.
All my comments appear the same size within the Calibre interface, including the book details pane. I'm not sure if this is a reflection of being on Windows XP or not. But that's why I never bother with the variations in the types of html coding that appear in the "HTML Source" tab. As long as things all looked uniform to me, I never bothered tweaking anything.