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.