There is no "best." Go with whatever you think it looks good. However, people in general tend to stray away from sigs that are too big or too small. Sigs that are too big are bad because they might slow the website down for everyone else since it translates to larger load times. Small sigs are considered "bad" mainly because I guess it'd be hard to make anything decent-looking in a small space. But, it's all subjective. Don't let other people dictate what you should do. If you're serious about graphics you shouldn't try to limit yourself in such ways anyway. Don't think of it as "will this be the right size?" Think of it as just "I think this looks badass, therefore I'll rock it." Anyway, to give you a more specific answer, a lot of people here like to go for sigs that are 400x150 for some reason. So if you really must use guidelines, go with that.