If your sms is over 160 characters, it will be broken up into 160 character chunks.
Googling, out of the box, I don't think the iphone has that feature.
Here's a quote from an iphone user in response to a similar question, "Apple wants the iPhone to have a simple and uncluttered UI that still delivers vital info. If a message is going to get split up on someone else's phone, I could couldn't care less. Adding more crap to the screen, like a character counter, just seems awfully silly."