a 20 or a 21 will work. If the bike has sloping geometry use the imaginary level top tube to measure it. If you feel stretched on a 21, opt for the 20. That will get you in the ballpark. An even more apropos measurement is the distance from the ground to the top tube, just ahead of the tip of the saddle. That should give you at least 2 inches of freeboard (because if the hill gets too steep you'll need to stand down, and it is way farther to the ground than on the level).

Ask me how I know.