On an open building shell a direct fired gas or diesel heater is the obvious choice: all the heat stays in the space and there is plenty of air supply anyway. Allow 20 to 30 kW for 400 to 600 m³, and more when doors open regularly.
In a closed shell, a marquee with an audience or a storage space, pick an indirect fired unit. The flue gases leave through the chimney, so no water vapour or combustion gas enters the space — essential during finishing work, screed drying and food storage.
For a fixed workstation, a terrace or a semi-open area an infrared radiant heater beats air heating: it warms people and surfaces rather than air that drifts away. No fan also means no dust being stirred up.
If there is a solid power supply and the space is limited, an electric unit will do. Check what the connection can handle first: 3 kW on 230 V, higher outputs on a 400 V supply.
Unsure about the output? The buying advice calculates it from volume, insulation and the temperature you want.