Well, one reason for me to register at ps3news.com was - don't lol - the absence of signatures. The clean and well-kept layout makes reading and posting a real pleasure. So, here are my 'votes':
1) Jr Members and above
2) No special idea here, but bear in mind that image-only signatures are easiest to verify and approve. Signature texts should be excluded in searches or selecting 'Search Signatures' would be mandatory. Links may link to a proper destination, several months later you'll find malware there. It would be sad to lose manpower from posting care to signature verification.
3) Maybe 75px for normal members, 150px for premium?
4) at least Contributor and above should be able to 'hide' signatures. The signatures might be spoilerlike fold in or out, a global setting for reading default could be then Sig-On or Sig-Off. It would be nice to have a button (beside rep' maybe) to fold out signature for a posting to have a peek when default Sig-Off is selected
5) for me it would not result in more postings, for others - no idea. Does male-enlargement really work?