How tall should I go?Yes, that's big enough. I would make it taller if you can
-Puppet
i do 816 watts in the same size with a 4" in-line fan, but it is also in a cold basement. temps are usually between 85 and 88
How high do yo think i should make the box if i did a 4x4 space?I've found that having a few extra feet of area to play with on the edges is damn handy. 4x4 would be fine but it's yer cab. 4 or 5 plants under a screen would be best if you are limited on height. I see a nice SCROG in your future.
If you are planning on cooling the light with a separate fan that is sourcing air from outside the grow space, then a 6" inline for regular exhaust will be plenty with a passive intake and circ fans going