Both file size (for the poor 56k'ers amongst us) and physical size - even me with my huge resolution has to scroll off to one side now and then.
(Also maybe restrict the number of lines of text too - although not sure if that can be done with vbulletin, some are ridiculous*)
*lists of equipment, including backups, with every shiny upgrade on a separate line spring to mind - maybe if these can't be limited, the owner of said sig can be "requested" by a mod to cut it down a bit?