For the decision which kind of heater and the required heat capacity the kind and size of the room that needs to be heated are decisive. The following general rules give you guides for the often required quick decisions - in case you have a heat requirement calculation from an architect at hand this one should be preferred.

For buildings and halls a general rule assumes a max. temperature rise (difference between the lowest anticipated outdoor temperature and the highest inside temperature) of 30 K (30°C). For other temperature rises you have to reduce or increase the heat capacity by 17 % for every 5 K (5°C)

For the calculation of the required heat capacity of green houses and tents different to buildings and halls not the room volume but the entire outer surface should serve as base for the calculation.

This simply because the heat loss of such constructions reacts proportional to the outer surfaces.