If you are using just one screen, I think you will find the small will meet you needs quite well. It has the advantage of less hand/arm movement to go across the screen and is big enough so small movements are not too “tight”.
For two screens, I find the medium works better. While the “throw” to go across both screens is long, it is not too long and quick. Since the screen area is so vast, you need a bit more tablet area for small movements.
The reality is that either will work. It is what you get use to. Just trying to give some food for thought.