For running with pressure the venturi doesn't need to help draw fuel so the size can be increased a lot. The obvious upper limit is when the engine won't run any faster.
If you have access to a lathe you can turn out whatever size you want. If you make a venturi it is a good idea to make the inside smoothly curved from the outside diameter around into the venturi. The inside should have an airfoil shaped section, but large enough to flow more air than the stock venturi. Anything to smooth the airflow and eliminate corners and edges along the way helps.
Inside the motor the front of the crankshaft should be partially filled and faired down nearly to the entrance to the crankcase using hi-temp, filled epoxy. Besides giving the air a smoother ride around the turn, the filler helps stiffen the shaft. Our crankshaft designs are about the worst possible from a fatigue/strength aspect. A hardened crank, especially on where the hole ends near the front of the cutout, with a big, chunk milled out of it with sharp corners is just saying "Break Here". So round and and smooth the corners and the inside too.
Don't start doing this kind of thing with a brand new motor. If you're looking for a racing type motor the ones with more than four ports in the sleeve are already tuned for max performance. Some of the car engines have to be detuned to swing a usable size prop on a plane.