Actually the size of the intake does not have to be twice the size of the fan.

If you want a higer velocity of air to move into the cabinet and give it a slight breeze make the intake about the same size as the fan.

If you want the air to move through more slowly then make the intake bigger.

Either way the intake will flow that same volume of air just at different velocities, as long as you don't make the intake to small.

Same concept with car engines that's why 4 valves are better than 2 they flow more velocity into the cylinder.
hoodrider Reviewed by hoodrider on . My Grow Cabinet... Almost Done..... This is my grow cabinet it is almost done. Thought I would post it for peeps to see. Dimensions 60 x 30 x 17" (152 x 76 x 43 cm) Top is a T5 2ft Balast 4 bulbs 2000 lummes each 2 warm 2 cool. Bottom Is the HOme depot Route. 27W CFL Plan to have 6 total. Top has a 5 " Computer style fan and the bottom has 2 5" Computer style fans 105Cfm fans. 30bucks each The temps right now stay around the same as the outside of the box. I got real messed up when I didn't make the Air intake hole Rating: 5