The smallest space heater I have ever messed with is 250 watt. A closet the size you describe would get too hot with a space heater that size. A 60 or 100 watt rough service bulb low in the closet would surprise you how much it would heat it up. If I was going to warm a closet i would look into building my own radiant heater from non-regulating heat tape. Get a 3 to 4 foot piece and wrap it around a large clay pot without over lapping the tape. Hook your heat source to a thermostat what ever means you choose. You are starting at 70 degrees so it wont take much. Most important part is experiment, do it right, be safe, and stay small with your heat source.