Way too big. I really love the image and I think it would make a great logo for a website or something. In fact, I think it would make a really nice signature, but not for a forum (more as an e-mail signature or something). That's because in a forum, especially in threads where you post a lot, the image just becomes too overwhelming and overbearing when it is continuously repeated. (As an email signature, for example, it only appears once at the bottom).
Maybe you could shrink it a bit and then use it? Maybe refrain to not too much bigger than 60 to 80px in height or something.