I generally try to make my signature not too big. I try to avoid using big images too, because decent html code can achieve a nice effect too. However, if anyone has problems with my signature please tell me.

I think a size limit would be okay, something like "max. size 150 pixels" would do. In the beginning it would probably need some work, but after a while most users will get used to it and not break it anyway. I've seen it work on forums way bigger than this one.