Minecraft wiki entry for TNT_cannons, associated text: The default definition of cannon size is how many TNT are in the shot or how far it fires. If you are talking about compact cannons, you might be talking about literally the size of the cannon (e.g. how much space it takes up) or if you are talking about cannons meant for mass destruction, you might be talking about the size of the shot. But usually, people mean charge mass.