another noob fan and filter size question!!

hi, ive yet another question that ive read and read to get the answer but cant come to a conclusion, my grow space is lx7ft wx8ft hx7.5ft, or 2.1 x 2.4 x2.3, ill have 2x 600 cooltubes, they will have there own venting, and ive 9 just starting to veg so need to get a move on to finish my "room", but cant decide what size fan/filter to get, im looking at ruck fans and either mountain air/or can filter(any comments) and a fan controller so if theres any one better at maths then me please let me know what size would be best,
cheers
cozz

please help!! ive come up with 3 different ways so far!, i think its lenght x width x height =? x20 =? but ive seen it done at x 10, and then lights ballast etc etc, im confused