If you're very strict about how large sigs get, you can set the limits in the admin options, but also make 100% sure the sigs use up a max amount of space by wrapping all sigs in a <div> tag with width and height defined to the max size you want sigs to be, and the "overflow: hidden" attribute, which will crop anything exceeding the width and height you dfine.
Now no matter what the user puts in their sig, be it a long line of text, or an img tag that goes larger than your div tag's defined size, it will be cropped. Keep in mind the content still loads, so if they put a large image on the img tag, the load time will be the same, but the forum layout doesn't get destroyed by it.
Can anyone tell me where I might put this code EXACTLY.