I am remodeling a bathroom and moving the fixtures. I have a couple of questions. The main soil stack is 3". Should I use 3" or 4" pvc from the water closet to the soil stack? I assume the stack vent going through the attic would continue with 3"? I have included a picture. I plan on venting the sink and bathtub/shower up the wall and tie back into the stack vent in the attic, is this acceptable? I will use p traps on the sink and shower and use 1/4" drop on the pvc.

This is the picture