The sharpen tool is a port of unsharp mask from Mplayer. The parameters are the usual ones for unsharp masking, The "size" means the size of the blur, and the amount is how much of the blurred version gets subtracted.
See this tutorial that describes how to use
- Alpha Manipulation>Blue Screen
- Composite Transition
- Crop and Transform>Pan and Zoom effect
- Sharpen Effect
- Alpha Manipulation>Alpha Operations