We currently allow 300 characters for the signatures, mainly to limit those people who don't normally use forums and are unsure of how signatures look. If those people were to spam, they have 300 characters to screw with, and if we added more, it'd be even more.
However, as an exception, I'll raise it to 500 for the time being. However, I'll have to say that if I find someone abusing the limit, I will lower it.
*Note: an alternative would be to create a signature in Photoshop with the information you'd like to show.