Hello there. I'm not an admin, but I am close enough. For signatures, the max is 600 X 200 pixels(Width X Height). If your signature is too large, then unfortunately you must edit or change it.
(I wouldn't recommend going with the max size, as doing so makes your signature appear to be gaudy...)
Edit:
Dang you Hsu... No wait, dang you 28 Kbps connection. While I had to check the rules, Hsu beat me by a screaching 6 minutes...:cry: