All of the current high end markers come with a stock bore size of 0.689 / 0.691 - I have found these bore sizes to be more then fine for playing.
The tighter the fit, the more efficient your gun will fire (not by much) - and is apparently shoots straigher. Buying a bore kit will actually take care of any barrel needs you may have if you actually want to invest in it. I would suggest sticking with a basic two piece barrel that comes bsic on all eclipse and dye guns. If you want to get a Barrel Kit (spending more then £100.00) I would suggest the Smart Parts Kit or the Empire Freak Kit. Both cost a fortune and I would personally say save the money and pay for more paintballs and entrance money.
Barrel length is another matter - I've seen tests shown that you only need the first 4" of any barrel for an accurate shot. Thus IMO falsifying the claim that the longer the barrel the straigher it flies. Again, going back to the standard set by the large gun manufacturers, 14" barrels provide excellent length in predicting where your paint will fly, as well as it being long enough to move shrubs or sup' air bunkers out of the way and poke your barrel through. 16" barrels give you the perception that your paint is landing where you're pointing it, but in fact has the same accuracy as a 14" barrel.
Its all personal preference on what you want to buy, but stock barrels on any Dye, Eclipse, Bob Long, Luxe's will have you covered.
Barrels are an investment that will depreciate quicker then the guns.
Hope this helps.