Growbox! Mechanics?

Ok so I have a 150 HPS light and I'm trying to grow 2-4 plants at a time. How big should my box be? Dimension wise? I have a fan but last time it dried out the soil. Solutions? How do I get the box well ventilated? I've already lost 2 fem. seeds.
I just need any and all info you can give me.
I really just want info on how to set up a grow box I guess. Mine was ghetto rigged last time so any knowledge in advance would be helpful.
Cheers Cannabis Connoisseurs.

2 plants from seed, a box about 1.5'x2'x3.5'h top it, tie taller branches to make even canopy covering the area or do a scrog, place an exhaust fan place a fan 50cfm with cooltube to the light in hot days hole for passive intake. get a good medium that retains moist but airs well, don't veg too long, don't harvest too early enjoy