First of all try it out on a paintball day. See how you find it. If its not as accurate as you'd like, maybe consider a 14". Like i said, test it out first. I was surprised at the accuracy of mine with the 11".
there is no 'cals' mate but its called a bore ? if thats what you mean? its up to you really depends on what paint you use e.t.c. not sure how much now? 85 or something i think.