For showers I always use a 2" waste pipe then I read the IPC code NYC has adopted and I find it confusing
"SECTION 417 SHOWERS 417.1 Approval. Prefabricated showers and shower compartments shall conform to ASME A112.19.2/CSA B45.1 or CSA B45.5/IAPMO Z124. Shower valves for individual showers shall conform to the requirements of Section 424.3. 417.2 Water supply riser. Water supply risers from the shower valve to the shower head outlet, whether exposed or concealed, shall be attached to the structure. The attachment to the structure shall be made by the use of support devices designed for use with the specific piping material or by fittings anchored with screws. 417.3 Shower waste outlet. Waste outlets serving showers shall be not less than 11 /2 inches (38 mm) in diameter "
"SECTION 417 SHOWERS 417.1 Approval. Prefabricated showers and shower compartments shall conform to ASME A112.19.2/CSA B45.1 or CSA B45.5/IAPMO Z124. Shower valves for individual showers shall conform to the requirements of Section 424.3. 417.2 Water supply riser. Water supply risers from the shower valve to the shower head outlet, whether exposed or concealed, shall be attached to the structure. The attachment to the structure shall be made by the use of support devices designed for use with the specific piping material or by fittings anchored with screws. 417.3 Shower waste outlet. Waste outlets serving showers shall be not less than 11 /2 inches (38 mm) in diameter "