Well, as far as the short thing goes, remember that Aang's a lot younger than Zuko; give them a few years to catch up so Aang can hit his growth spurt, and that factor should even out a fair bit.

And while being raised by Iroh should mean Zuko's picked up some tips on charming the ladies IF HE WAS PAYING ATTENTION, don't forget that Aang has actually BEEN FEMALE in multiple past lives; think of the potential if he can tip into all those millenia of memories!