In general, the return air flow is equal to supply air flow, but if there are any exhaust air in space like bathrooms, electrical room, etc. then the amount of return air would be equal to amount of supply air minus exhaust air.
For sizing the grille, refer to the grille manufacturer catalog and select the one with NC-30 or 35. The air velocity out of the grille shall be 500 to 700 fpm (foot per minute), however the NC shall not exceed for the allowed amount for space per ASHRAE recommendation. For regular office space, school and similar applications the NC shall not exceed 35.
Calculating the thickness of the insulation needs a complicated heat transfer calculation but generally speaking, the 1" thick duct insulation is sufficient for the indoor ductwork.