Actually, I had signatures off for a long time untill a few weeks ago for that exact reason. I agree that large images and lots of text shouldn't be allowed, but I think there should be a enough "room" to make it what you want...within reason.
Maybe you could make [img][/img] tags add to the length but not [url][/url] tags?
* musicalmidget thinks he's asking a little much. 
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