Thanks fattyre, that's the answer I was lookin' for. So is this space larger on a 20'' frame? Since the head tube is longer (by how much?) does it take a different fork? Thanks again./QUOTE]
Head tubes are 6 inch and steerer tubes are 7 1/8 on 20 inch frames. Truss rods are longer too. And even longer truss rods for locking forks.
Also note on 20 inch frames where the lower tube intersects near the head tube.
From the top of the BB shell to the bottom of the straight bar is about 14 inches measured along the seat post on this bike:
1941 whatever model it is. Similar frame to the one you pictured with the curved down tube.