whats the difference between short, medium and long rear mechs(apart from the obvious)

Just the obvious. The greater your spread of gears, the longer cage you need. CX bikes with a 38-42 double and 11-25 will get away with short cage, for a 22-32-44 triple with 11-34 cassette you'll be going for a long cage

ta

If your buying a new one, and you like Shimano stuff, I can reccomend the shaddow mechs, especially on a hartail, as they are just so much quieter on rough stuff, as the mech itself cant hit the chainstay.

and dont bother with the long cage Shadow, it is stupid long and sits so low it defies half the alleged point of being "shadow" ie. out of the way of rocks. Good mechs though, nice direct cable routing gives reliable shifting and as tinsy says they dont constantly rattle against your chainstay.