I get the feeling NorthIrish wants you to listen to him!!!
Anyway, my advice, if you can save for a set then do that. It's mainly dependant on the size of the paint you shoot, so if you can get a good paint to barrel match then you're gonna be more accurate. Shooting paint through a barrel that's too big for the paint
IS going to make a difference to the way your paint flies.
Lengthwise, as said before, personal preference. Me, if I'm playing at the back I prefer a longer barrel (18") as I find it that little bit more accurate & easier to wrap from corner to corner. But if I'm playing mid or front then a 12"-14" does the job fine.