Size - take care!
Got a "Large" size red kong for our labrador and, although he loved it briefly.... he chewed the end off it! (the smallest "orb" of the three which make it up). We've since replaced it with an "XL" black one which has resisted so far. I've no reason to think there was a defect with the one he killed, but no matter how strong Kongs are, if your dog can get it far enough back in his jaws it will be vulnerable.