My understanding is they were first used to help pilots in "pursuit aircraft" overcome the force on the elevator when pulling out of a dive or other high speed maneuvers.

Evan,
The size can be pretty much anywhere within the parameters listed, it's the amount of travel or throw if you will that governs the force applied.

"The boost tab should normally have a chord of less then 25%, and an area between 4 and 10% of the control surface. About 6% seems to be a good compromise. If very large angular deflections are required of relatively wide chord ailerons, the boost tab area should be close to 10%. I discovered this recently as I fine-tuned a new sport design that was intended for fun-fly aerobatics. The boost tabs can be made full span without using impractically small chord just reduce the throw considerably. I've used full - span boost tabs on rudders; at 10% of the rudder chord, the throw needed for optimum balance was only about half that of the rudder itself. It's best to start with too little movement just to be on the safe side and gradually increase it until you have solid control response. " Carl Risteen