I think there are two ways you could do it, VT:
1) to strip it off all signatures, including existing ones, open admin/functions.php and find:
Code:
$post[signature]=bbcodeparse($post[signature],0,$allowsmilies);
Before that add:
Code:
$post[signature] = preg_replace("/(\[)(size)(=)(['\"]?)([0-9]*)(\])(.*)(\[\/size)(\])/siU", "\\7", $post[signature]);
2) to strip it off all newly added or edited sigs, open members.php AND register.php and find in both:
Code:
$signature=censortext($signature);
Before or after it, add:
Code:
$signature = preg_replace("/(\[)(size)(=)(['\"]?)([0-9]*)(\])(.*)(\[\/size)(\])/siU", "\\7", $signature);
Cheers,
Bira