Depends how much you have to spend matey:
for cheapness, try the DYE excel, they come stock with the e-blades and just rock considering how much they cost
Mid range: boomstick or ultralite, you can buy backs for both seperately, allowing you to slowly build up a multi bore system
High end: Pipe barrel kits, powerlyte scepters, empire kits. Loads of these out there, however try to avoid a freak, they are a pain to align and if you break a ball in there you're screwed!!
as for bore, try a .689 to start with, most paint will fit down it, but if you can't afford a few bore sizes, buy a big bore, while it may be too big, all paint should fit down it, although you may have a coupla problems regarding consitsenvy over the chrono.
As for length, 14" seems to be good for all positions.