I don't think there should be any specific rule about that... no signature has bothered me so far. If someone gets a huuuuge signature, then one of the mods could just ask him to remove it, no big deal.
I don't like putting rules if there's no need to, what is this, north korea?
Just kidding. If it becomes an issue and people start to use huge signatures, then yeah.
If it becomes an issue, at least a width limit, because if a signature is too large in width it can make text spill out from the normal size of text boxes. A height limit is probably not necessary because people can just scroll down.