This is a fair question that is almost impossible to answer as each individual feels heat differently and what is comfortable for one person, may not be for another
We can give you statistics such as: 1 kW of heat will heat approx. 3 square metres of outdoor space or perhaps 4 square metres indoors, in a church or barn etc.
In the real world there are many factors that come into play and influence the nice convenient statistics.
Ambient temperature - a 2 kW heater will feel very different when the temperature around you is 15° C to when it’s 5°C
Wind Chill – On a windy day, you will feel colder to begin with so again will need more heat to compensate for this and feel comfortable
Colour of Clothing - Infrared heaters are essentially a sophisticated type of light and you feel warmth if you’re in that light. Different colours absorb or reflect light differently and my twin wearing a white tee shirt will feel colder than I do wearing my black tee shirt as dark colours absorb the light.
Distance from the heater – we generally advise that the 2kW heater should be mounted 2.3m – 2.5m above ground, but if you’ve had to mount higher than this for any reason, you will need more heat to feel the same benefit.
Individuality – I (the writer) am constantly surprised at how cold my elderly mother feels wearing several layers of clothes while I’m sweltering in just a shirt!
ALL THS MEANS that the above statistics remain “accurate” as a guide, but increasing the level of flexibly is justifiable and if budget allows it may be worth considering a more powerful heater, a heater with remote control that lets you adjust the output, or perhaps multiple heaters with individual control.