take this for example . if you hook up a 1 inch garden hose and turn on all the way the water will come out kinda slow, but if you hook up a 1/2 inch garden hose the water will come out faster and it doesnt hurt the plumbing. i had a 6inch inline exhaust fan hooked up to a 4 inch duct and the air flowed harder it was good. remember to have an fresh air intake on your box so you wont burn up the fan. hope that answered your question