Measure the distance between the center points of the top and bottom rings, along the broad face of the shade.
Not sure what type of FITTER you need? Click on the Fitter Styles link under the Lamp Shade Options menu bar to the left.
"The Drop" describes how far down the fitter is relative to the top of the shade. A 0" drop means the fitter is flush with the top. A 1/2" drop means the fitter is placed 1/2" below the top of the shade. And so on.
The visible hardware (spokes, etc) can be any color you wish.