2x3x6=36 cu ft. Most 4" fans will do 200 cu ft/min. (With some losses associated with connected filter and ducting).108 cu ft/min will give you 3 air changes/min. So the 4" fan would be my choice even running 2 lights. If you're also inquiring about a future light addition, I would suggest searching Amazon for groplanner gp1600m. It's a 150w manual dimmer variety. Made by Hyperlite. Claims to have over 600 leds but I haven't counted. Currently $100 on Amazon with a 50% off coupon so $50 for a solid 150w light. These are quality lights. By far the best bang for the buck in lighting. Coupons come and go so if you don't see a deal, keep checking back. Pocket the savings and use it to help avoid cheaping out on other needs such as ph pens, nutrients, etc.