Different rotor diameters changes the feel and characteristics of the motor and while they will have some influence on runtime the biggest factor would what winding motor you are using and the gearing you run.
casterracing EX-1 Pro, SP Silver Arrow
TOP Racing Scythe, SP 17.5, SP GT Pro 2.0
TOP Racing Scythe, SP 4.5 V2, Hobbywing Xerun 120SD 2.0
TOP Racing Scythe, SP 13.5, Hobbywing Xerun 120SD 2.0