the math is
LxHxW devided by 3 and that is the minial amount of air u need to move
now add on 10% more for the cfls........the hps it is 15 or 20%
if your intake is hot (above 70) then u need anouther 20% more speed
thatshould be near the right CFM what ever CFM that is u try to match it ......if nothing is close then go for the higher choice and use a varible speed controller on the fan
the smallest is a 80 cfm 4 inch booster fan .....(the speed controllers do not work on boosterfans)
the smallest inline fan is 4 inchs 170 cfms or 180 ........the speed controller will work
if money is tight get the booster fan ............if u have the spare cash get the inline can fan ..........u can get the inline fan with a combo carbon filter to clean the air before u eject it outside the area