A good 6" is fine. I cool 2 1k lights in my veg room, and they are cool to the touch.
BEST DEAL -
http://www.businesslights.com/valueline-inch-435-cfm-centrifugal-inline-fan-p-2045.html
Just try to keep your turns to a minimum, are use metal ducting for long runs.
However - If there is even the slightest chance of expansion later, go ahead and spring for an 8" now !
PS It is ALWAYS better to push air though a system that is going to be ventilated
to the outside. If you have ANY air leaks in your system, you'll be sucking "That Smell"
out for the world to smell...