Personally, I don't consider it good design to have a field named "SSN". Have a field for "Identifier" or even "ID Number" and make it varchar(30). Don't you want your clients to ever have non-U.S.A. customers? Don't you ever want to have clients from other than the U.S.A.?

For example, I live in the U.S.A. but my insurance company does not even know what my SSN is nor the SSNs of any of my family members. I do this for philosophical reasons but also because it greatly reduces how many forms and people end up with my SSN, which I feel reduces my risk of becoming a victim of identity theft.

My ID number for insurance is assigned by the insurance company and starts with an "N", not a digit (though it is only 9 characters long).

I pick 30 for the length because I remember when New York did something rather stupid. They changed their drivers' license numbers to be an encoding of identifying characteristics plus a sequence number (the first digit designated your eye color, etc.). That resulted in a 21-character driver's license number. This turned out to be very inconvenient for my employer who was selling software with a 20-character field for ID Number (which was rather larger than required by the drivers' license numbers of any state prior).