What is the procedure for the calculation of the diameter of the drive shaft of a BAJA buggy?

Yes, there is a procedure in our design book.

If you are going to build a BAJA buggy you have to go reverse that is by applying Market availability of drive shaft. Whatever you calculate is impractical or not available in the market. So the best procedure is to do a market study and then decide. Do reverse engineering. The overall calculation is based on torque and speed. Always remember your theoretical knowledge is important but not always useful for practical purposes.