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The scale is what you have set for your document under "Units", and it's very important that they match the scale of your project, or else the lights will not behave as expected. I'm still trying to wrap my mind around the power units and often seem to have to use a light more powerful than I would have expected. I, too, have not noticed any difference between the various types.

I manually mesh to allow myself full control to optimize each of the model parts, but I know many here rely on the render mesh settings (I find doing so slows my screen refresh down on complex models).