It all really depends on whether you will run pressure or suction. If you run suction, you should start small (about .218″ or so) and slowly increase the size until the motor won’t draw fuel. The shape should be an actual venturi shape. I ground a cutter for the lathe to do this shape. The location of the needle valve is somewhat critical with suction and it should be placed below the smallest diameter and the hole should be placed angled down, at about a 10-15 degree angle.
If you are running pressure, then the hole can be much bigger and the actual shape is not really critical, for an engine like this. Start around 1/4″ and open it up, looking for a rpm increase. When it stops, you stop….
I have tried many shapes and sizes, but most of it just doesn’t do much with a lower rpm engine….