I am using the $15 heater from Wal Mart as mentioned in the second post right now in a space about that size, it works great. I have it on LOW and it uses 750 watts. It's not hot and you don't want to blow a breaker! I would HIGHLY recommend though, it's a must in my opinion, buy a thermostat. Dont rely on the thermostat on the heater. Search ebay for "Greenhouse Thermostat" and you will find one, your local hardware may also have one but at first they won't know what you are talking about.It keeps the temp within 3 degrees.
The built in thermostat with my Wal Mart heater kept the temp within about 12 degrees.