1) Your signature may not exceed 400 pixels in height
For those of you who have no idea how much that is:

As you can see, that's still plenty of space.
2) Even if you have 400 pixels you *CAN* use, it doesn't mean you *MUST* use them. Do not fill your signature with crap. If everyone decides to abuse the 400 pixel limit, it will be reduced again.
Anyone with a signature too large will have it deleted, and a warning issued.
Also, all the posting/board rules apply to your signature. So that means no advertising, no flaming, etc.