This post pretty much sums it up nicely on the rules for signatures:
http://www.namepros.com/the-break-room/669757-signature-question.html#post3893387
Also here is a copy of the post as well
...Thanks Maximum!
Rules:
- Should not be more than four lines in length (current length-check code, this is hard to do now, unless your sig was already longer before code change....is buggy, but you violate rules rather system lets you do it or not).
- No images (codes stripped-out).
- No use of a font size larger than 2.
- Adult links/domains are not allowed - Keep the smut out.
- No referral/affiliate-link/parked domains or the like (if you profit at all from it just being clicked-on, don't put it in signature)
- No links to other domain forums.
Basically: no more than four lines, G-Rated, no bigger than the "size 2" text, no profiting by just getting it clicked on, and no using it to lead NP visitors elsewhere (other domain forums)...Follow those rules, and all should be fine.
If your domains can follow that...post 'em