The higher the value, the more high frequecies will be diminished. In Marshall circuits, I have gone as high as 180pf and couldn't hear a difference from the stock 47pf cap. I could hear a 250pf cap
The capacitor reduces the gain of the amplifier at frequencies above its normal operation due to phase reversal through the output transformer. To my knowledge its not a "tone shaping" component and RussB's experiments bear this out.