I've been seeing a lot of gigantic signatures lately. I've even sifted through a thread about having a giant sig and being all proud of it (or something along those lines). Mahz, when he enabled sigs a looong time ago (they were not originally enabled), announced an actual size limit, but didn't hard-code it so people could have a little bit of leeway.
I don't know the official length limit offhand, so I've got an easier one for you. Stick your thumb on your monitor (unless it's greasy/covered in Cheeto dust). If your sig is longer than your thumb, it's too fat. Either slap some spoiler tags on it, or put it on a diet.