API 12K latest addition will give guidelines to determine the diameter of the firetube. The length has to do with the flame length as you want to have several feet past this distance at high fire. This distance determines the main leg of the firetube and if it is a return type then two times that distance will give you the total length for surface. You may need more surface for forced draft burners that can fire into smaller diameter firetubes than what is used for natural draft. The length of the first leg will also determine the overall length of the water bath shell. Since the shell is open to atmosphere then the only pressure you have is the height of the liquid. The main concern there is not so much the shell itself but the end plates. The thickness of them is more guided by the height of the liquid and the fact you have force on a flat plate. A good book to have is the 'formulas for stress and strain' by Roark & Young.
I hope this helps.