How is the Correct Surface Load of a Heating Element Determined and Verified?
Maximum watt density of a heater, all other things being equal, is primarily determined by the technology of the heater. A swaged ceramic type is capable of a much higher watt density than a humble mica heater because the internal nichrome element is held much closer in temperature to the 'skin' of the heater. 

If he internal element runs too hot for its diameter, life is compromised, sometimes dramatically. It's probably determined in a fairly empirical manner- heaters have been around for a very long time.