You can run 20" wheels on the new Model S, even though it's not an official option in the settings. Lots of people have done it without any problems. The 19" and 21" wheels have slightly different diameters, but Tesla's systems are usually pretty tolerant of minor differences.
My advice would be to use the 19" setting if your front and rear tires are the same size or if the rears are slightly larger. If the fronts are larger, use the 21" setting. The tire pressures are the same for both sizes, so you can use that pressure for your 20" wheels as well.