The dimensions sound fine. I'm a new grower, and I initially bought a 4" inline fan, but found out that for my grow box it was just overkill, and is too noisy. So I'll just be using it for odor control now and then. For air intake/output with a growbox, all you really need are some computer-type fans. Try to find some with cords or power adapter, bghydro sells some. What I did was got four cat litter box air freshener things at Target, which have small computer fans in them. I removed the computer fans and attached them to PVC piping, two for the outlet and two for the intake. The fans are quite weak, so it would be a good idea to get the type that bghydro has, although these are bigger so it will be more of a challenge to attach them in a manner that allows for the light to be blocked.