Exhaust fan size

My room is a storage room...Its 3.5m long, 1m wide and about 2.5m tall...Only gonna have 1, maybe to ladies going at once. Running 1 600wMH for veg and a 600w HPS for flowering...What size should I get? 4", 6" or 8" fan?

3.5x1x2.5 = 8.75m^3. This is the volume of your space. Fans are rated by the volume of air than can displace in a given time. Factors like carbon filters and intake/exhaust port sizes can inhibit the ability of a fan to operate at their rated volumes. Check the ratings of some of the fans you're interested in to find out how much air they can displace.

Your room is 309 cubic feet so you need a fan that is rated at least 309cfm a good 6" should flow around 350cfm
http://www.amazon.com/gp/aw/d/B002JQ14F8/ref=mw_dp_sim_ss4/178-4855711-2710615?pi=SL500_SY125