Ok first question
Yes the size different from paint ball to paitn ball in the sme bag is very small. we are talking in the ball park of about ±
.002 of an inch for fresh tournament grade paint while old cheaper paint will very as much as ±
.01 just in the same bag.
This difference will cause a big difference in how consistently the paint will hit the same spot or area because of a few things.
1. If the paint is too small it will "rattle" down the barrel and fly out of the barrel in slightly different direction. It may be less then 1° but over a distance it adds up. Also the gap that is there means that the gun will need more air to get the same velocity.
2. If the paint is too big it will break in the barrel and cause all kinds of problems for ya.
Also Although all paint balls are called 68 caliber does not mean all paint is going to be exactly 0.680 inches in diameter. I have seen paint that ranges from 0.671 all the way up to 0.699 this is just across the board from alot of brands
One other thing that will affect the size of the ball is the weather
No you don't need to change the barrel for every shot. Doing that will make a pump player seem like the fastest gun out there.
With a Flatline the best thing to do is try to find a paint that has a shell that will survive being shot threw the barrel and still break when it hits the target.
Although the flatline has a nice flat trajectory when you shoot it looses alot of velocity to get that extra distance. While lobing a ball to hit the same target may seem like a pian they can have a slightly higher velocity when they hit the same target and you can use a more brittle paint that would have broken in the flatline.
On average a most players use a barrel that is any where from 12 to 16 inches in TOTAL length. This will very from player to player based on how they play. Also when looking at a barrel there is some thing called "Effective length" (EL} which is the length of the barrel from where it starts to where the porting starts. So I copuld have 2 barrel that are both 12 inches long, One has no porting while the other has porting that starts half way. The first one has a EL of 12 inches while the other only has 6 inches.
As for bore size there are three that I use the most out of my freak kit; the 0.689 0.687 and the 0.684
The best way to size a match the barrel to your paint is to be able to gently blow it threw the barrel with out it rolling threw or getting stuck.