Diameter, and the mount
Rotor size is leverage. A larger rotor gives the same lever pull more stopping power and copes better with heat on a long descent — but the frame, fork and caliper adapters all have limits, and exceeding them is not safe.
The mount is the other half. 6-bolt rotors bolt to six threaded holes on the hub; Centerlock slides onto a splined hub and is held by a lockring. They do not interchange without the correct adapter.
Rotors are also a wear item. If yours is below the minimum thickness stamped on it, or visibly grooved and blued from heat, it needs replacing regardless of how the pads look. Bring the wheel to the shop floor and we will measure it.
