A bit of random clicking later I discovered that you can change the size of a drawer by grabbing the little white space than runs around it, giving much more room for editing the bookmarks. I suppose this is pretty obvious, but somehow I have managed to miss it all this time!
[robg adds: Yes, this is a basic hint. But if you're relatively new to OS X, it might not be obvious that drawers are resizable. This goes beyond Camino, of course, and works in Mail, OmniWeb, and other apps that use drawers. Oddly enough, in searching the Mac Help files, I couldn't find any references to resizing at all; I'm not sure how you're supposed to learn this stuff other than through experimentation if it's not documented...]

