This is not difficult. To size the return air requirements for the basement (or any room), figure the area of each supply duct in the room and add them together. The return-air grill you use must have as much free area as the total area of the ducts supplying air to the room. The basic principle is to equalize the air pushed INTO the room with the air sucked OUT of the room.
Also, make sure you seal the insulation flex properly and use long zip-ties to connect the ducting to the exhaust/intake boot. You do not want your return air sucking insulation back into the system. You also want a sealed system which has no leaks into or out of the rooms.