What you need to do is get a copy of 'Model Submarine Technology' By Norbert Brueggen, available from Traplet publications-
This I refer to as the submariners bible. It covers just about every area of constructing a working model submarine, short of DIY hull construction.
Folk from a non-technical background will find it a tough read, but if you're serious about scratchbuilding model submarines, it needs to be digested and understood.
Determining pressure hull strength is well covered in that book. The dimensions of your cylinder are usually dictated by the shape of the hull.
One last thing, if you do purchase the book,and I strongly recommend it, don't build any of the electronics. For one they are seriously out of date now, and secondly they're riddled with errors.