JT's fairly on the money in terms of how it affects riding - shorts do tend to make bunnyhops and general tricks a bit easier, but for anything that you need to kick into (e.g. gaps, taps, sidehops) longer frames are usually a bit better. But yeah, with your particular situation just literally get any reputable frame. You haven't had a trials bike before and so you've got no real frame of reference for whatever you get, so whatever you get is going to feel weird, if you see what I mean? If you've got nothing you're used to, you'll get used to anything. You just learn on that frame and ride in that style, pretty much. My first trials bike was a freakishly short original-style Onza T-Bird. Progressed from that to frames that gradually got longer and longer and they fitted my style a bit better. But fundamentally, you can learn anything on any trials frame, which is why the top riders can jump on whichever frame their sponsors give them and still ride well. Nothing really makes up for just getting out and practising, so just get whatever frame you can get ASAP, then get out and start riding!