Return air is a same amount of the supply air and the duct size for return and supply are the same. If fresh air (outside air) comes through the AC unit & economizer to the space and then significate part of that will be exhausted through the restrooms exhaust or other places exhaust systems, therefore, the return air amount would be a supply air minus the total exhaust air and the return air duct can be sized a bit smaller than the supply duct. For small amount of the exhaust air, it would be better to ignore that and size the return duct equal to the supply duct.
The fresh air (outside air) amount should be per local code or ASHRAE tables (see How to estimate a required outside air (fresh air) for each space)
For static pressure calculation in ductwork see How to calculate the static pressure .
