Many of the Fast Subsurface Scattering shader's parameters, such as radius and depth, are extremely distance-dependent. This means that if you create the effect on an object measured in millimeters, and assign than object's material to an object measured in inches, the effect will not look at all the same.
The Scale parameter on the Advanced Settings tab of the Fast Subsurface Scattering shader's property editor is a simple multiplier for distance base values. Set the value to the appropriate conversion factor when switching between measurement units.