It's best to get an idea of how big your cat is by measuring its height at the shoulder. If he/she measures over 20 inches tall, then you'll need a large carrier. Smaller breeds usually fit into small-sized carriers. For example, a 10-inch tall cat needs a medium-sized carrier, and a 12-inch tall cat requires a large carrier. It doesn't matter which carrier you choose as long as it fits well and allows plenty of air circulation.  Also, you need to take your cat's measurements from the tip of the nose to the tip of the tail while it is in a standing position; after that, it is recommended to add four inches to those measurements. Doing this will give you the approximate size of the carrier your cat needs. Remember, the carrier must not be too big or too small. Typically, only one-third of the total area of the carrier should be used as headroom because cats love to stretch out their bodies.