Well, the problem is that everyone has a different opinion on what an appropriately-sized signature image is. Some people don't want sig images at all, some don't care, and the impact of a signature image is largely dependant on the viewer's screen resolution, something we have no control over. As far as I can remember, this is the first time in the history of the forum that this has been an issue, and I don't think it will be a big enough problem in the future to impose signature size rules. Besides, there's nothing server-side that can check that, and the staff members of this site have better things to do than checking pixel sizes of every signature they see.
All we need is for people to be more concious of what they're posting to their signature, and the problem should correct itself. I removed some images from the worst offenders that I was made aware of, and if you see one that you think is too big, or you're curious about your own signature, feel free to PM any of the admins or moderators about it.