I make beer can Coehorns from 4" diameter stock and bore to a depth of 3 7/8". Since this is about 3/4" shorter than a 12 oz can, I use the shorter 7 oz cans, which actually fly better since they are closer to spherical than the 12's are. Truly spherical shot work even better.
The powder chamber size depends on the material you use for projectiles and the kind of range you are looking for. I find that concrete filled cans will break up at launch if you use too much powder. This would not be true if you were to use steel or cast iron shot. I would stay away from lead shot in this diameter in a Coehorn style mortar. If you made a seacoast mortar with its thicker walls, lead shot would not worry me.
My beer can mortar is proportioned like a seacoast mortar even though it has a sort of Coehorn appearance. It was my first artillery piece and made from photos instead of plans. It was made from 6" stock which makes it very heavy (getting heavier every year) but it is quite stout and I will shoot any reasonable projectile from it. Its bore length is more like 2 cans long (about three and a half calibers) which gives it greater range than a Coehorn of the same bore. At the Montana shoot, it launched a zinc round shot out of sight on about 250 grains of Fg.
So I can't really answer your question definitively. A long narrow chamber deals with light charges better than a wide one but a wide one allows heavier charges without making the rear wall too thin. Compromises, compromises. You need to decide what your operational requirements are first.
