I find a jump bike with 24" wheels is much easier to control and learn for dirt jumps. But on here 26" seems to be more in vogue.
The less front suspension the better 80-100mm set up hard (you don't want to suck up the lips).
Frame size – it doesn't need to 'fit' you as your not going to be really pedaling or sitting down that much, just make sure its small so you have plenty of stand over, you want the saddle super low. I prefer a longer TT but that down to personal pref.
Oh and its got to be steel.