This depends on a few factors concerning your dog: their activity level, size, and age. The larger the dog, the bigger the kennel so they can move properly and have space to run and play. If a dog is more active, it is also recommended to give them more room. Furthermore, an older dog usually needs less room because they are more docile. As recommended by Make and Build Dog Stuff, less active dogs only need around 50 square inches of space whereas larger more active dogs need around 150.