Whiskey has an excellent point in mentioning, that you don't need to be like 7'2 to possess an effective post game (although it might help^^). Besides having some moves, post play has more to do with being able to overpower your opponent and having a wide frame to shield the ball from him. Barkley is a great example, but even in today's game, players like Bonzi Wells and Ron Artest are known to be effective in the post although they are relatively small for the position they play. And there are also a lot of examples for players, both overall good as awful ones, with length that never developed a reliable post game (KG or Dirk Nowitzki anyone?).
But I have checked out some Al Jefferson videos on the net and although most times you see him backing down to protect the ball (that's what post game is about), I have seen a lot of scenes now where he can be compared to the players defending him...and I'd say he is no way as tall as the listed 6'10 but indeed more like 6'8, maybe 6'9 if you are generous (difficult to judge a person that moves).
But as Whiskey has pointed out, size alone makes not for a great post player.