I've made sigs all the way from 200x75 up to 500x200 - the latter is HUGE and ends up with messages being all sig and little content.
450x200 should be more than adequate for anyone.
400x100 is a nice size:
The above are EQ2 sig's I made for me and my girlfriend, 400x100 and both under 46k (JPEG 100% quality). By not being very tall they don't distract from the forum much, but there is still enough room to get some fine detail (not to mention some Sun Tsu quotes) in.