My screen resolution is 1680 x 1050. But I don't think it's an issue with that. Possibly it's an idiosyncrasy with the browser I'm using: Opera 9.24 on Mac. Browsers are *so* inconsistent. Or perhaps it's something Mac specific. Dunno.
I'm using Firefox and also set for 1680X1050. The sig in question looks ok and fits within the border.