If you are going to use a travel crate, which is a great idea for smaller and medium-sized dogs, sizing is important. Like many other aspects of sizing your dog, the weight matters less than the height and length. A thirty-pound dog can be shorter than a twenty pound dog, depending on the breed and age. A good rule of thumb for any dog or animal is to allow them to both stand up and turn around comfortably in their crate. Both plastic and wire crates should have a liner, bed or even a towel along the bottom of it to prevent sliding or pain. Once you have gotten the height and length of your dog, you can find the dimensions of a crate that will suit your needs. You do not want to oversize the crate either, as smaller dogs may attempt to relieve themselves in one side if they can get far away enough from it. Finally, make sure that you have each animal in their own crate. Unless you are transporting very small dogs, such as a litter of puppies, you never want to have dogs sharing a crate during travel due to space limitations and stress.
There are a number of reasons to crate train your dog. Most puppies and adult dogs feel more secure in a small, enclosed den-like area, and young puppies are especially eager to find a safe place. A carefully-chosen crate and proper crate training can help your puppy discover that a crate is a safe, happy place to spend the night. A youngster should be introduced slowly to the crate when possible and not left unattended longer than the dog is able to "hold it" for potty training. But crate training, when it's done right, can give both you and your dog a feeling of order and security in a complex world.