I can tell you my experience with using a venturi. First off you need enough water flow to make it work. As Kelly noted, scale up the diagram. If you would use the scale in the diagram with stream water, debris will plug the 3/8 hose barb and the party is over. I have found that a 2" venturi is about as small as you want to go. A 2' venturi in a 3" pipe will work. You can use a much smaller air inlet then the size of the venturi, I used 1/4" hard plastic tube in a 2" venturi. Its all about water flow not just water pressure. You can see this effect whenever you drain a sink or a bath tub, at some point air sucks down into the flowing drain. This is the swirling tornado of air you see. Obviously the more pressure and the higher the water volume, the more air you can inject. If you can build the venturi out of clear pvc you can see it working.

First find out what size pipe you can keep full of water with your current flow. Use the largest pipe size you can. Next build a venturi to reduce the pipe size a bit cut your large pipe and insert it. Drill a small hole in the venturi for the air inlet pipe. Cut the end of the air inlet pipe at an angle and face it down stream. The tip of the air inlet pipe should be sticking into the venturi. Your venturi can be on shore where you can play with it to get the best results. The angle of the venturi vertically or horizontally makes no difference

This is actually easier to make work the one would think. It's a fun project to play with also. Because the water pressure (head pressure) and flow rate are unknown you cannot apply any formula for venturi size percentage that will work most efficiently in all cases. Experiment and have fun with it.