What would be the best size of disc brakes of 20 inch wheels?
The size of rims do not determine stopping power. You could put whatever you want for rims on the car and as long as you have good tires, your stopping distance will be the same. What changes stopping power is weight in the car. Most aftermarket rim/tire combinations are relative to the OEM rim/tire combos so not to worry. Add a couple hunderd pounds to the car and stopping will be harder, increase the performance of the car and stopping will be harder. Sure big brakes look good with large rims but most people with realitivly stock cars with large rims only upgrade their brakes for looks. Unless you are running more than 50hp more than OEM, than you NEED to upgrade.
Brembo makes all kinds of Big Brake Kits for Mustangs, take a look at their website. you can get multiple 4 piston calipers with drilled/slotted rotors in different sizes to suit performance or just looks. You are only governed by your wallet.