Instructions
Find out how tall and long the puppy will be when fully grown. If the dog is a full bred breed, this information should be easy to find online or in a breed-specific book.
Measure the height of the puppy from the top of its shoulder to the floor. Measure the length of the puppy from the base of its skull to the base of its tail. Use a temporary cage at least six inches larger than those measurements.
Add 6 inches to the maximum possible height and length of the dog to determine the size of a permanent cage. The 6-inch rule should make the crate large enough for the fully-grown dog to lie on its side with its legs fully extended. A puppy can be placed in this larger cage immediately and will just have extra free room until he grows unless you choose to insert partitions.
How to Choose the Right Size Cage for a Dog or Puppy
Your puppy's cage or crate will be one of the most expensive things you buy for it, so you will want to buy one the puppy can grow into. It will be a den, a potty training aid, and the thing that saves your shoes from falling victim to a teething or anxious dog while you are at work. Although some dogs might initially be skeptical of the crate, with a little gentle coaxing and positive reinforcement, even the most timid dogs will be happy to oblige.