I ride a 19" modern MTB off road. For a flat bar bike I like a 59-61cm center of seat tube to center of head tube top tube length. On my vintage Mongoose, that means I can ride up to the 23" frame. I also have a 23.5 High Sierra that fits the bill. My 88 or 89 RockHopper is a 20" frame, but apparently has a longer top tube as all 3 measure the same.