The size of generator that you need to run a portable heater is at least a 2,000 watt generator or higher.
Most portable electric space heaters use 1500 watts on high but you need some extra power so you don't damage the generator or lug the generator down.
It's better to have a higher wattage generator than you think you need.
I would certainly go with a 2000 watt or even 5000 watt generator to run the portable heater as you can also run other things instead of just the portable heater.
A portable generator in the 5000 watt or higher range is a good investment.