Bore sizes range from about 0.679" to 0.695", and some barrel systems (like Freak, Powerlyte Scepter or Sanchez) use inserts to achieve this. You ideally need to match bore size to the paint you are shooting for best efficiency/accuracy and to minimise breaks. Your marker will need the right thread to fit as well - impulse/ion style.
A good post by gee tee there, although I will mention that this is a point with a bit of controversy surrounding it. Many people arguing for and against bore matching in favour of either under boring (barrel slightly smaller than paint) or over boring (barrel bigger than paint) for various reasons.
Punk Works (
http://www.punkworkspaintball.com/) have done extensive testing on the matter and seem to favour under boring.
Personally I've usually over bored in the past as then I know even the most miss-shaped ball will get out of the barrel in one piece.
(paint will change shape and size as it's stored and opened to the atmosphere)
I'd say get something around the .689 size as it is a happy medium that will underbore large paint and over bore small paint but never be too extreme that you notice any major draw backs.
As for thread, as GeeTee mentions anything Ion or Impulse threaded will fit, but I'd mention that these have also become known as smart parts threaded after the shocker NXT was released with them (shocker SFT are threaded differently)
I reaslised I dont have a clue about barrles 2 piece 1 piece? thread? bore size? im not sure what any of this means and i dont want to buy a barrel that wont fit my gun so if you kind people could explain for that would be awesome!! 
