I could hazard a guess and say that the bolt is likely in the 4-6mm range, but to find out for sure, you can get thee to a hardware store (I dunno which country your in) and size it by comparing to other bolts, sizing it with a nut, bolt & thread gauge (a hardware store will have these for sale but you can use one on the shelf for measuring your built without buying the gauge), or ask a salesperson to help you size said bolt. All I can say with certainty is that your bolt is metric. If you live in the US, Ace Hardware stores almost always have a large selection of bolts. FYI, who makes the seat post clamp and/or your bike?