Please give tell me standard of diffuser size and CFM we can use for design of air conditioning system and also return air grill size and cfm range .?

the best selection method of diffuser size is the manufacturer catalogue, based on the noise level set by the designer you can get the FPM of a specific size of grill or diffuser, once you have the FPM and grill area its easy to get the CFM (FPM*Ft square)