Welcome to MP!
Make sure that you do not get a duct booster fan, which will not work for your application. You need a centrifuge type fan, like Dan K has linked (change the xx in hxxp to tt). This is the type fan I have in my 3 x 6 closet.
You may well be able to use 1 fan for both cooling your light and air exchange as your space is not large. You will also need a fan speed controller. hXXp://www.amazon.com/Hydrofarm-ACSC-Duct-Speed-Adjuster/dp/B003URBQ1M/ref=wl_it_dp_o_pC_nS_nC?ie=UTF8&coliid=I3GQ2IEB0M09EY&colid=1HHYPCL51QYFW