Whether you need a bigger clamp for a carbon rail saddle depends on your seatpost. The normal size for metal rails is 7mm and they're round. Most carbon rails are 'oval' (often 7 x 9mm) like this one seems to be, but the rails are still 7mm wide - so despite the rails being taller if your seatpost's clamp is the type that has plate at the top and a plate at the bottom, then the chances are it should accommodate oval rails as well as standard round ones.