To put it on par with real life (not that anyone writes letters any more) but it you did write a few lines and then signed it with a huge scrawl that was larger than your whole letter, it would be ridiculous. Large signatures are distracting - the purpose of a forum is communication, and if you are peering in between huge blocks of pictures to try and find what someone has actually written, then, well... you're doing it wrong!
500 x 89 by the way....