How do I calculate how many / what size of DAR heaters are needed to heat a given area? Is the calculation the same as for convector panel heaters?

Yes, it is the same calculation as would be needed for a convector panel heater.

See https://www.dimplex.co.uk/room-heating-calculator where you can perform a sizing calculation to work out what size of radiator you may require.

Applicable models: 

DAR050 DAR075 DAR100 DAR150 DAR200