The size of the playpen totally depends upon the size of your dog. Select wisely a playpen that is suitable for your dog according to his size. If you buy a small playpen while your dog is large then there would be no possible space for him to play and walk or even he can escape from it easily. For small dogs 30 to 36 inch long playpen is enough while for large dogs you have to buy a playpen which is 42 to 48 inch long.