I used to use a 250 watt in a similar space... obviously you're not going to be able to utilize the entire length of the closet because it is narrow, but you have like a 2'x4-5' area there still and could use a cool tube or a rectangular air cooled reflector. Inside needs to be flat white or coated with mylar that goes without saying.
If I was growing in such a space today and could vent it efficiently I would use a dimmable 400w. Get a movable light hanger and you're golden, then you would be able to use the whole closet.