Altering size of emission object without messing up brightness of light in scene To create a light source using Cycles, I create a plane (or other mesh shape) and set the strength until it looks good.  When I scale that light source, say make it three times bigger, the total amount of light increases.  Highlights blow out and the scene sometimes goes almost all white. So, I reduce the emission shader's strength. 

It seems that the strength is interpreted as luminous watts per square meter.  Is this how it works? Wouldn't it be better to be able to alter a luminous object, to control softness and depth of shadows, without changing the overall brightness contributed by that source to the scene. Is this possible to do this easily, and am I understanding what is happening correctly?
Set the emission using a driver



See this answer for the basics of setting up a driver.

Result