No size difference between the inlet and outlet. Two holes have the same area and can only allow so much thru.
Need to know how many BTU's you need to dissipate, and the dimensions of the enclosure.
The home made ones are nearly impossible to work with due to the various sizes, shapes and locations of components.
As has been mentioned before, a fan is better than no fan at all.
And as also been stated many times, it all depends.
