For me, some of the glimpses he's shown up forward mean you have to explore his ability there initially. Star forwards are simply harder to find - if you have a guy who could potentially be one, it'd be remiss not to investigate that potential. The key is, if not suited, and as others have said, let's not have an extended period of trying to make him something he's not. It'll be clear enough after one season of extended opportunity whether it'll work or not.
Two other things:
1) Why people determine a players position based on where he played junior football baffles me. Francis' best AFL position is flat-out unknown - I would suggest even to the club. It will become clear with time.
2) Height isn't the be all and end all as to whether any player should play key position. Leap, reach, timing and body strength are all factors as well.
Is Francis a key position player? Who knows - see point one above. Time will tell. Speculating re this on the basis of where he stands in the 190-195cm spectrum seems a tad silly to be honest.