size wont matter, its the size of the "chokes" or venturis inside them that matters, im no expert but you probably need something with 32 or 34mm chokes, changing them is expensive so try to find out what size you need a buy a set of webers with that size choke installed, other than that stay away from "42" webers as the air delivery is not suited to small motors, 40 or 45s will be fine with the right sze choke