A dog training crate should ideally be large enough for your dog to stand up, turn around, and lay down in. You certainly don’t want the crate to be too small as your dog will be uncomfortable. But the dog crate can be too big as well. You do not want the dog kennel to be too large because then the dog or puppy will have space to go potty on one side and sleep on the other side of the dog crate.
Make sure when you buy the dog crate that it will be the right size for you dog when he is fully grown not just puppy size. If you get the dog crate when your dog is still a puppy, you can get a panel that divides the kennel and allows your puppy to grow into the dog crate.
You can buy a dog training crate in either plastic or metal, but each has a more appropriate use for your dog or puppy. The plastic type of dog crate is more suited for travelling. The are lightweight and allowed on most airlines. The metal wire dog crate are more suited for your house. They tend to last longer, usually have more room and better ventilation. The metal dog crate is more of a long-term domicile for you dog or puppy.