3'X3' square foot is the common size limit for a 400 HID light. In a space that size, I can fit four flowering female plants, in 10 inch pots comfortably. The more ceiling height you have, the easier it is to cool.

A small fan will probably work in the winter/spring time, not in the summer. You probably won't be able to close the closet door, the heat has to go somewhere...:rasta:...and you need a fresh air source.