Updated - March 2015
The basic rule is this: its not the width of signatures that bother us so much...its the height of the signature. I only have a laptop to use and its sad to say I've seen signatures take up the entire height of my browsing screen at times. That's not ok, and will not be permitted on this board.
Therefore, it is recommended that all members keep their signatures small. Nobody needs a huge signature, nor should the board members have to continually scroll past a large signature.
The recommended size is 100 pixels high. You can make something bigger, but I strongly discourage it, given the explosion of smartphone browsing and small computers replacing the desktop in many cases.
Look at the yellow box below: its 200 pixels high by 600 pixels wide. That is the absolute maximum your signature can be. Again, the recommended size is no bigger than 100 pixels high, so try to keep it half of that box if you can. If your signature is too large, you will get a PM from staff asking you to shrink it.

Some additional tidbits:
- no image should be larger than 100kb in filesize
- please avoid animated gifs (they are often tacky and distracting)
We don't want to be signature police, but its now time that we're a well-established board and community to look neater and more professional. There's no need for your signature to take up a large area of the page when your message says "Good luck!" or some one sentence. Not everyone's going to be happy with this, but it needs to be done. If you fail to comply, we'll contact you about your signature via PM, and you'll be given two days to fix it, or we will resize your images for you.