The weld between the run pipe and the branch fitting has to be a CJP with a reinforcing fillet as noted by Shane. However, the fitting reinforces the run pipe to compensate for the material loss of the hole. The code does not list different size welds based on the pipe thickness or the design pressure. It is as Shane described. The manufacturers qualify the branch fitting and they define the physical size of the weld. Often the fitting will have multiple bevel angles. Typically the CJP has to be filled to the top edge of the first bevel with a reinforcing fillet. Other designs where the fitting is nearly the same size as the run pipe, things can be more complicated because you can not use a reinforcing fillet. It is important to know how the manufacturer requires the fitting to be welded for the fitting to function properly.

If you are asking the question relative to the socket weld, i.e., where the branch pipe slides into the socket, each piping code has different requirements for the size fo the fillet. It can be a small as 1.09 times the branch pipe thickness to 1.25 times the branch pipe wall thickness. You have to know what pipe code you are working to and if you are working with B31.3 Process Piping Code, you need the service class to know if the socket fitting can even be used.

Best regards - Al