Clamps generally only come in 3 sizes:- 28.6, 31.8, 35.0 (not 100% sure of last one). This is because the seat tubes tend to have external diameters of the 3 tube sizes in increments of 3.2mm/ 1/8th".
Thus the clamp for you is either 31.8mm if the seat tube is steel or the next one up if the tubes aluminium