a 250w or 400w will be ok in there if you have a great deal of ventilation and can keep it open most of the time or use an inlet fan too
id go for at-least a 6 inch fan if you are going to use HPS in that space ..
i have made a few of these type box's before the problem is the shape, they are tall but have very little width, so the plants can never spread out they grow very narrow and upright,
a screen is a must to stop the colas from shading each other
i found it best to grow a nice sativa dom strain in my wardrobe type box because that makes use of all the height with nice slender long colas .. g13xhaze kali mist c99 are good strains for this .. rather than an indica strain that will fill that space with leaves and not make so much use of the height and normaly likes to grow in a wider space/shape
you could split it in the middle and grow 2 levels in there, that would be more complicated but would maximise the yield you could have 125-250w cfl on each level grow plants 12/12 from seed or flower some clones early and keep them to a single cola each, and do a mini SOG on each level keeping them 1.5 to 2ft high or whatever your restrictions are