What size should a lavatory P-trap Be?

4 inch or 110mm NO less then 11/4" and normally 11/2" depending on the code your using Most installations are 11/4" INLET and 11/2" outlet used in conjunction with a reducing slip nut (11/4 -11/2) 4" is the MAIN House trap or in some cases a strorm trap depending on the area that is being drained