Disc brakes installation
Rotor size is unrelated to wheel size, but your fork will have a maximum rated size. Any larger than the rated size will put you at risk of a (literally) fatal breakage. Disk brakes dump a lot of torque into the fork. Have a look near the calliper mounts and there might be advice on maximum rotor size.
A bike of that vintage will probably want 180mm or 160mm rotors. A nominal configuration would be 180 front &amp; 160 rear.
Take a look at your hubs. Do they have 6 bolt holes on the non drive side to attach rotors? If not you need new wheels. Unless you already know how to build wheels, don’t bother with replacing hubs. Seek a pair of disk ready wheels.