Kent,
There is no single answer. It depends on power consumption, air-flow, maximum ambient and available space. A thermal solution will be needed. Thermal modeling is highly recommended. There is a KeyStone Thermal Guide. Pull it from: http://www.ti.com/litv/pdf/sprabi3. you can pull thermal models for the K2H at: http://www.ti.com/litv/zip/sprm603. We do not yet have models for the K2E but it will have similar 2R resistance numbers.
Tom