Cd you should check with your town's building inspector just to be sure you are in compliance.
As long as the room has a door and a window (egress out in case of a fire) - to the best of my knowledge, there is no minimum size a bedroom must be. If this is in a basement, you should definitely check with the town.
............closet, door, window, bed - voila = a bedroom!
Keep in mind that carbon monoxide detectors are required within 10 feet of any sleeping areas, so if this new "bedroom" is away from any other detectors, you must add one in the vicinity.
Prudential NJ Properties