I would also like to give a warning about tennis balls, or balls that are the same size as tennis balls.
They are JUST the right size that most dogs in the same size class as a GSD (labs, goldens, etc. ) can swallow them and have them block the airway. DO NOT let your dog play with tennis balls.
You may notice that the Kong brand ball that is about the same size as a tennis ball has a hole in it. The next size up does not. The hole is to allow a dog to still breath if it should happen to swallow the ball.
If your dog DOES choke on a ball, don't try to get it out from inside the mouth. You will likely just push it further in. Put your fingers on either side of the underside of the dog's lower jaw, and push the ball towards the mouth. Chest thrusts (Heimlich maneuver) have also been known to work, but are difficult to do on a dog that's conscious and struggling.