You should pick a playpen based on your dog's age, height, and weight. Young puppies won't mind smaller playpens and will still enjoy their surroundings. As your puppy grows, you might want to switch to a playpen that has more panels and is a teensy bit larger. Secondly, the playpen is your dog's leisure place. Puppies need lots of rest during the day, stretch, or jump around out of excitement. The playpen will be their place for that and so, it should be spacious enough for them to be in.