what kind of fan to use?

hi,i am hoping to grow in a box which is 2 ft wide 1.5 ft deep and 4 foot high.i am gonna have 2 plants and hope to try the scrog method.my main question is what kind of venting will i need.i will be using a 250 mh for the vegging.i thought about using a bathroom extractor fan at the bottom of the box to bring in fresh air,and cut a hole at the top of the box for the exhaust,does this sound right? i am hoping to start work on my box soon.please has got any tips for me.

put the fan at the exhaust hole not intake. you need negative pressure not positive. get the correct cfm fan

thanks bonz,what does cfm mean,i prob sound dull.would a small bathroom extractor fan be ok?as my box is only small.

also is it possible to fit a dimmer switch to the fan to be able to adjust the speed.

cfm meens cubic feet per minuite. multiply depth x width x hight. 2x1.5x4= 12 cfm. i think my basic bath fan is 50 cfm. you will need a bit extra from the 12 cfm for the back pressure when you add a filter of some kind