Thoughts:

I think the problem is less tall images (e.g. Radium's current sig is 180px high, and while big doesn't actually take up that much space) and more tall sigs (e.g. Drmoo's current sig is approx. 270px high).

A suggestion would be to say what the max height of a sig should be. Enforcing such as a hard-and-fast rule obviously requires a lot of effort, but it could be used as a rough guide (something like "sigs should be no more than 300px or 10 lines tall").