I think we need some hard limits to both height and width. For width, I see no reason anyone needs anything wider than, say 1024 px. For height, around 250 or 300. We can set those easily under the Members tab in the Control Panel.
EDIT: Not sure what I was thinking. With the signature in a wrapper, as Alex has said repeatedly, the image will resize, so worries about horizontal scrolling are gone. So, that leave just height, with to me is a worse issue since it makes you have to scroll like crazy. But since Alex has limited that, I'm changing my position and saying let's let it ride for a while - see if any members protest any signatures.