Required drain size for double sink and tub

What is the required drain diameter for a double sink (back-to-back) with a tub downstream?

Thanks for your help.

pipe

It depends on how you do the piping, but normally 2".

On the second floor of a two-story house, two lavs are at the beginning of a drain line. They are connected with a double sanitary tee and are branch vented with 1 1/2" pipe to the stack vent. The 1 1/2" drain runs 2-3 feet to the soil stack that serves the second floor. Note that a watercloset drains directly into this soil stack as well and that the soil stack is also branch vented to the same stack vent that the lavs are vented to.

A tub is being added, and the drain needs to connect to the soil stack without taking too many bends. Can the tub drain connect to the existing 1 1/2" drain line coming from the double lavs?