I'd get a 6" to exaust and a 4" as an intake, but you don't have to as long as the cfm rating is considerably higher on the exaust. I've had issues finding them at lowes locally but I think it's just this area. You can go on the lowes website and find them or just get them from HTG which is probably cheaper. Those inline duct fans are great for a small place like yours, but they're hard to find it seems like. I don't think they're used very often or something.