400 CFM is a 6in inline fan if i'am correct(dont buy a cheap one), but first it depends on where your growing at like which location upstairs, downstairs, basement, shed and etc
also its winter so u got a winner on your side helping you out, what i think is with the 6in fan it would not be a problem with heat cause i measure the size 6.5L 6.5W 6.5H is pretty good size(also wanna ventilated very good) but 6.5H would be a problem you only have a limit of space to adjust your light height
18in 400w
24in 600w
30in 1000w