400 or 600...? 400. You are going to have trouble keeping it cool though.
I would guess you will need to keep the doors open during lights on.
In anything smaller than a 3.5 x 3.5 I would just use LED.
I tried running a 400 in a 2x3x6 (converted armoire) a while back and quickly figured out that it was not going to work. I ended up using some 2 foot fluoro on two levels to do my veg growth. Now I use it to dry half my garden at a time.
If you are dead set on HID... you will need separate air cool and exhaust circuits. Don't try to get away with the carbon filter on top, light below and one exhaust with passive in, it just won't work out well.
You will want a LARGE fan for air cool and/or a source of cool air to pull over the hot bulb.
In the winter, outdoor air could keep it cool during lights on. In the summer you will have heat issues.
2x3x6 is LED/fluoro territory if you ask me. A 400 hid in there will be a fucking headache but you could pull it off with large fans and cool intake air.