A typical 1500 watt electric resistance space heater draws 12-13 amps and provides about 5000 BTU of heat. A 12 gauge extension cord will be OK if you can keep it about 50' in length.
A portable heat pump/AC unit, like I use in my coach during the Winter travels, supplies about 11,000 BTU's of heating(twice what an electric resistance heater supplies) and draws 1250 Watts or 10.8 AMPS of electric. This could easily use a 12 gauge extension cord. The portable unit also has a fan to move the heat throughout the coach.