How big should the chamber be however?? 4 times as big as the volume of the barrel and the "magazine" area??

As big as you can make it, 4 times the barrel volume is a good start

And when you say chamfer..do you mean cut the edges of the barrel at a 45 degree angle inwards so that the golf balls sort of "slide" into the barrel?

Yes.

And when you said a 2" golf ball chamber..Are you saying that the golf balls would just all be lined up and pushed out in a row??

Yes, but they will be pushed out sequentially and emerge as a burst, because since they're in 2" tubing, the air can flow past them and push the ball in front of them.

Look at it this way, if you put say 5 golfballs in your regular spudgun barrel, then the air acting on the surface area of one golfball is trying to push out the weight of 5 golfballs - resulting in an instantaneous mass of golfballs at relatively low velocity.

On the otherhand, with DR's design and mine, the air is pushing on one golfball at a time, resulting in a burst of golfballs fired at a much higher velocity.