Originally Posted by JSanders
160 characters on many carriers. It varies.
I was about to say that the other characters are for addressing information; but now I'm reading two conflicting wiki's (one that says 160+ and the other says it's 160 inclusive -- but now I'm leaning towards what you said).
However, I also read something I didn't know that explains something: while some messages do get split into two, it's usually the phone software that does this, most carriers truncate the message at 160 characters if the software doesn't do the split. I often wondered why some txt's seemed... well, half finished, heh.
Edit/Add: In any event, the signature method given, using autotext, seems best if you insist on signatures for SMS, but an auto-signature might be a bad idea for the reasons given.