From w:en:Wikipedia talk:Signatures#Length section:
Right now, the maximum length for a custom signature is 255 characters (or 255 bytes?). This is already enforced by the software.
However, if your custom sig is {{subst:my-sig-template}}, then right now, that's "25 characters", even if the template contains the entire text of Homer's Odyssey.
My question for you: Do you all want me to ask for them to start checking the length of signature templates? There's no software need for this (that I know of), but if this is something that editors want, then I'm willing to make the request.