I have a 8 inch for 2 hoods and exhausting the room and a 6 inch for intake although a 4 inch would have been better your exhaust should be bigger than intake,my room 5x9, i would go with 4x8 heres my set up
remember that you lose cfm on your fan rule of thumb anything you connect it too will lose about 150-200 cfm example
8 inch inline 745 cfm
8 inch inline cfm connected to 2 hoods and carbon filter about 400 cfm
dont get a fan controller i would get a thermostat controller instead