What, you want me to get off my ass and find a ruler? Oh, alright. Heh, the top shelf is 10.5" of useable vertical space, the shelf itself is 0.5" thick, then beneath it the growing space is 53" high. And yes, I think you could probably use a 400 watt HPS with a cooltube, but you'd have to be a bit more careful not to do what I did and over-veg the plants too long. Might be better to go with two plants too, but that depends on how big you let them get.