Im guessing that you are from the UK
Best chamber would be 110mm / 11cm pipe, It is harder to find pressure rated PVC in the UK so if your going with un pressure rated then go with ABS much safer.
Barrel I would go with would be probably 40 - 50mm barrel.
Bigger is always better. There is no replacement for displacement
If your serious about building a good cannon, and not a cheap one with bits you can find, I can put you onto a good few PVC suppliers in the UK
Just private message me because it is late here and I can't be bothered to go searching at this time (12:00 PM/)
Dave