I live in san francisco and am adding a lavitory sink I plan to connect to current shower drain with a wye and loop vent through same what size pipe do I need and what is the best way to support it ? Sink will be 4 1/2 feet from shower drain thank you tallgrass:o

Trap size on a lavatory is 1 1/4". pipe size should be 1 1/2"

A vent must be at least 6" above the flood level rim of the fixture