That's a bit of a tricky question to answer because it depends on the type of engine and what it's to be used for as well as the size prop it's driving amongst other things.

Too large a venturi (for whatever load the engine is driving) just means the airflow through it will be slow which gives low fuel draw. A slightly smaller venturi will speed up the airflow without really restricting the volume of air so the revs would stay much the same. Go smaller still and although the speed of the airflow may keep rising it'll begin to restrict the volume and then max revs will start to drop.

My G51 has a .282" venturi and a .156" needle valve blocking most of it and I run it at 8000 on the ground in a 4 stroke with a 12x5 prop. It's got a few thousand revs still in hand but I certainly don't want it going there. If I used a smaller venturi to limit the max revs then it'd likely be in a 2 stroke at the 8000 ground revs and that would make a mess of how it runs in the air.

So venturi size really depends on what you want from the engine.