In all fairness, you can buy all the parts you like, but if you haven't got a grasp of core principles, you run into difficulties when putting them together to make a working sub.
Those principles have been covered many times before. Back catalogue of SCR's, Norbert's book on sub technology, plus a few online articles.
When figuring out WTC and ballast tank size, it's really useful to have a working knowledge of Archimedes principle with regards to displacement, densities of various materials. When working with ballast systems, especially those that compress air inside rather than vent externally, you should also understand how volume alters with pressure and vice versa (Boyles law). e.g. halve the volume double the pressure.
A WTC should be large enough to provide buoyancy for the tube weight itself plus all the equipment you put inside it. In addition, allowance for ballast in the keel and remember that glassfibre hulls are denser than water by about 30-50%, dependent on the glass content in the laminate.