Years ago Ebbw Vale RFC had electric heaters suspended from the roof of the seated stand. They were above your head and you could feel some heat coming down. I went there once in mid winter and it did help a bit as you could feel some radiated heat. I don’t know what wattage they were but there were several of them and not that big.
The layout of the area you want to heat will be important. How sheltered is it? When in the steel industry I remember standing near furnaces and you could feel the hot/cold difference between opposite sides of your body. Once I was next to a 150tonne ladle casting into moulds, my front was warm but the wind was blowing snow onto my back through the open end of the casting bay 30 yards away.
Managing the space your heater has to cope with will be important with windbreaks. Being able to move them to suit helps. Could you hire a heater to see how efficient they are? Remember, too, that the colder it is the more clothing you wear as well. Having been to Northern Norway in winter and experienced outdoor heating ( whilst drinking hot drinks!) the combination of clothing and heaters varied. Oslo harbour had ice in it but heaters, clothing, and gluhwein kept us glowing. Hamburg Christmas markets have outdoor heaters.
Don’t look for huge heat output to allow naked outdoor seating!
