We make them ourselves from hard maple. They are 36″ (89cm) long, 1″ (25mm) thick, and the working end is 4″ (102mm) wide. What can I say… All my woodworking tools are still marked in inches rather than centimetres! All edges are rounded over to avoid splintering. The length was chosen because of the shipping surcharge that is incurred if the parcel is longer than 100cm. Unfortunately parcels to the USA, where the limit is still 36 inches, still get the surcharge.
It is best if your beating surface is a little below elbow height. That way it is easiest to make the stick land squarely on the surface. By turning the stick you can use either the flat face for less pressure or the edge for more. You can also beat pulp in a pail on the floor by pounding it with the end of the stick held vertically, as long as the pail has a sturdy, well-supported bottom.
In our shop we also use one of these as a general stirring tool for large containers. It also works well for mixing the pulp in our larger vats.