that is way too subjective. How big sigs are depends on your screen resolution as well. Plus what seems too big for me, could be ok for others. There would be non stop complaints if this was implemented.Its simple!
One Rule: Don't make your sig so big that it will annoy other users.
If this rule is broken your sig will be instantly removed.
You can review the sigs before you delete them anyway, and its just common sense to tell if a sig is appropriate or not.