Can I use 6" duct for venting out a basement? I will have four runs coming off the main duct, two in the 12 x 22 bedroom, two for the pool table open area - roughly 20 x 40 - with two vents per duct run, and then one more for the work out room 16 x 20.

Thanks

You can use any size duct you want but you should really calculate how much air volume you need then pick the closest size duct to install.

A six inch duct will supply about 85 CFM where a 7 inch duct will supply 125 CFM and 8 inch will supply 180 CFM . Now it depends on have much CFM do you have from the supply, so instead of square footage we need to know the amount of air you will have and the size of duct you will be using and if it is flex duct or metal, flex duct looses some CFM compared to metal duct.