My greenhouse is 8x16x10, so it is just about the same size as yours. I'm in Zone 6b. I have used an oil filled radiator type electric heater. It has a thermostat, is reliable, no danger of fire, and if I remember correctly, cheaper to run than most common space heaters. I keep it on the floor in the center of the greenhouse and have a 16" oscillating fan up higher to keep the air circulating
