This is one of those obvious tricks I stumbled upon by accident: if you click on the dot between the URL entry box and the search box, you can drag and resize both of those fields.
[robg adds: Hmm, guilty of overlooking the obvious here as well -- never tried that, and I'd wanted to have a slightly larger Google box in Safari!]
Mac OS X Hints
http://hints.macworld.com/article.php?story=20030517152809422