This size refers to the dimension across the top which determines which type can be fitted to which suit. You need to measure the ankle of the suit where the new seal is to be fitted. If you measure this flat across and then double it you get the circumference. Ideally the suit should be 1-2cm bigger than the seal which allows the latex to very, very slightly stretched on which gives the best finish. If the seal is bigger than the suit you will end up with creases in the latex which will probably leak.
42cm Small 0009, 42cm M/L 0008, 36cm Small 1010, 36cm M/L 0010