Tanner has being doing tunnels since he was a pup. Regulation agility tunnels can be between 22-26 inches tall. Tanner is 32 inches tall. However, just like in the dog door post, Tanner doesn't seem concerned about the size difference.
You can see that I throw a toy for Tanner to keep him driving straight through the tunnel. If I had an adult dog Tanner's size that I wanted to teach this behavior to, I would start with a hula hoop. I would want to dog to be comfortable to scrunching down to get through the hoop before I moved on.
The next step would be to have them go through a tunnel that is completely scrunched up and more like a thick hula hoop than a tunnel. I would gradually lengthen the tunnel while keeping a high rate of reinforcement up for the dog. Keeping training sessions short so that the dog is never tired of the activity is also important!