Do you mean kennel, as in a kennel run outside? Kennels outside - the bigger the better. You want the dog to have room to sleep/play/drink on one end, and go to the bathroom on the other, and have shelter. My smallest is 20' x 16'. I have a medium to large breed. If you're asking about crates..., then you want a size that the dog can stand up without ears touching top, turn around easily, and lay down comfortably. If you have a specific breed of dog, most crates in stores have a tag that lists what popular breeds fit into each crate. So if you have a Golden Retriever sized dog, then you'd buy one that says that it is suitable for a Golden.