If you check
https://bitcointalk.org/index.php?action=profile;sa=forumProfile you can see the global signature restrictions.
Signature:
Signatures are displayed at the bottom of each post or personal message. BBC code and smileys may be used in your signature. Signatures taller than 42px will be cut off.
That is 42px, not 42pt (42px are 32pt afaik).
As for restrictions based on ranks, check the sticky topics in this section.
- Newbie: No styling (including links) allowed. Max 40 characters.
- Jr. Member: Links allowed. Max 100 characters.
- Member: Unlimited length.
- Full: Color allowed.
- Sr. Member: Size allowed
- Hero: Background color allowed
