Ah i see, i just got this from a HVAC website, it's estimates on the cfm ducting can hold.
4" Duct 40 cfm
5" duct 60 cfm
6" duct 100 cfm
7" duct 150 cfm
8" duct 200 cfm
9" duct 300 cfm
10" duct 400 cfm
12" duct 600 cfm
14" duct 900 cfm
16" duct 1,400 cfm
18" duct 1,800 cfm
20" duct 2,300 cfm...