Size is the obvious and primary considerations that must be given, but there are other factors just as important when it comes to picking out the correct carrier for your dog or pet. Setting size aside for the moment consider the other factors.
First how will the carrier be used? Is it strictly for travel? What type of travel? With a dog that doesn’t like to travel or one with an active disposition requiring mandatory down time you may find it useful to use the carrier inside the house as a bed or kennel as well. For smaller pets it’s easy to buy one with more room, for comfort as a kennel that can still be easily used for travel purposes. But even for larger pets the proper carrier still has plenty of room to use your carrier as a bed. Unfortunately Great Danes, Mastiff’s and other gentle giants are too large for most carriers. Those dogs should just be treated like a horse or human, it’s your preference.
The purpose of most carriers is securely transporting a pet from one location to another. But what type of travel will be used to accomplish this transport? If your travel includes motorcycle or other recreational type vehicle you should probably look for carriers specific to that type of travel. Something that is more suited for open air travel and motorcycle travel is best with a carrier that is hard sided, padded inside and crush resistant to some degree, for ultimate protection.
How about air travel… do you plan to use the carrier for traveling by air? If so you need to pick a carrier that will slide under the seat in front of you for a pet you plan to bring into the cabin. Getting the pet in cabin is more probable for small pets and service dogs. But the space under the seat in front of you is not the same on all aircraft. For this reason carriers of a soft sided nature work best. The carrier needs to be flexible yet sturdy enough to avoid collapsing in on your pet! If you plan to send your pet through as cargo due to your pet’s size, travel destinations or other travel restrictions, you will need a hard side carrier.
As well you need to consider options offering some level of climate control. Cargo storage at 30,000 feet in the air can be extremely cold in the winter and hot in the summer. For cold weather travel the carrier needs to be able to retain the animal’s body heat by perhaps covering it and using warming pads or put a sweater on your dog. During warm weather travel consider getting a pad or bed that can offer cooling like cold packs and if possible a small fan for air flow. If the travel will be long enough to require food and water, space within the carrier may be needed. Add this consideration to the size of the carrier, or purchase a setup that will mount to the inside of the door or carrier walls. Above all your pets comfort during air travel in cargo is paramount. The stress related to this type of travel can be tremendous on some pets. You must do everything you can to make the animal as comfortable as possible to reduce the stress as much as possible, all the while keeping safety as the number one concern. Before purchasing anything for air travel check with the airlines to learn about their requirements or restrictions so you can make a fully educated assessment of what you really need for a safe trip. You may find driving to be a better option over cargo travel!
If your travels will require lots of walking you may want to consider a carrier on wheels, mostly for your own comfort. There a number of options available from stroller types to pull along types. This can also be a great option if your travel companion is smaller like a Chihuahua or Yorkie. Or short-legged companions like Dachshunds and Corgis. All these guys struggle to keep up with the human stride and stamina and surprisingly enough carrying even a 6 pound Chihuahua for extended periods of time can be draining and tough on your arms! With that in mind, whether you give them, and yourself, a hand by pulling or pushing them or even carrying them in a front pouch or backpack, is a personal choice best suited to meet the needs of your lifestyle and physical abilities.
If you plan to use a carrier inside a car or truck, picking one that can be used with a seat belt will offer double protection for your pet during a collision or other type of accident. When the sole purpose of the carrier is for inside a vehicle you should also give consideration to using a more comfortable and travel friendly option such as a lookout seat or console seat. With the use of a harness and restraining strap a dog car booster seat such as these offer your pet a safe alternative from the carrier with much more comfort when traveling. So if the carrier seldom, if ever, leaves the vehicle, consider the pets comfort when making a choice. Would looking out the window make him or her happier when traveling? Providing a non-enclosed space may help the pet who suffers motion sickness. And just the overall feel of a plush well-padded seat and sides is sure to make any animal more comfortable.
When the main function of the carrier is to provide an in-home safe haven or kennel for your pal you should consider a size large enough to accommodate the dog when standing or laying stretched out, have some room to move about and possibly an area for food and water and/or a potty pad area. If your dog is small enough there are extremely large carriers available to meet these requirements. Having something fully enclosed like a carrier can provide quarantine, confinement, privacy or added security and warmth when needed by simply throwing a sheet, towel, blanket or rug over the carrier. Also worth noting is cleaning and disinfecting is easier with a hard side carrier when using it as a kennel.
So even though a carrier can be used for multiple tasks or serve multiple functions, consider the primary use and from there determine your size. No matter what the purpose a carrier is being used for, at a minimum, it should accommodate the animal with enough room to fully stand up, turn around and stretch out when laying down. Exactly how much more room is best to have, beyond the minimum, depends largely on how much time will be spent in the carrier. Put yourself in your pets place. If you have to spend x amount of time in a minimum space, how much extra room would you want in order to maintain a fair level of comfort for the amount of time required? Scale that answer to the size of your pet and you should have the proper size carrier.