try setting the pedal to seat length at 110% of your inseam that should be close to the correct riding height then check how the bars feel. if you go for the 20 make sure the bars have sufficient height adjustment. for sure this will not be a flat foot setup, but then where you ride makes a difference. i am 5'11" and ride a 23" road bike and a 22.5 mtb on the road. i have no problem with the height but my feet only touch the ground at the start and end of a ride.