Can I change rotor size with disc brakes?
Rotor size is determined by the brake mount adapter.
So everything else ie. the caliper - stays the same. Only the Adapter changes.

You must check your fork manufacturer for the maximum size of rotor the fork can be safely operated with.

Also check frame clearances (at the rear) for enough clearance for the size of rotor you are selecting.

Going up rotor sizes is specifically for more braking power.