9/16" Big Strong tubes are the most durable and have the most volume and wall thickness. Regular 1/2″ tubes are good for most ChewFactor 3 chewers. Start here (Regular 1/2"), and if your child needs just a bit more to chew on, move up to the 9/16" Big Strong tube. If less is okay, or if you want to decrease the amount of drool, move down to the 3/8" tube.
3/8″ Slim Strong Tubes are a good fit for younger chewers and feel softer, though the hardness (durometer) is the same. Alternatively, Big 9/16" and Regular 1/2″ tubes feel harder when chewed. Deciding between the Regular and Slim sizes depends on how strong your chewer is and their preferences. If you don't know where to start, try the Slim Strong Tube and increase the size if it is not strong enough.