The dog shown on the treadmill here is too big for that machine. The “run area” needs to be as long as your dog’s stride at the speed you plan to move him at + extra at either end for the random adjustments your dog will make as he moves.
Here are three basic ways to determine the best dog treadmill size for your dog (or, in a multi-dog household, your largest dog):
This is the easiest and least precise of all measures. A dog who is low and heavy may need a much shorter run area than a dog tall and lean. A good example is my own dog, Pip. She is in the leggy and lean category. She is 35 pounds which means she is under the max size for a small treadmill but she needs a medium one by all the other measures.
Here is a fairly simple assessment: Measure from the back of your dog’s neck (about where his collar sits) to the base of his tail (where his tail attaches to his body). Then multiply by 2 if you plan to walk/trot your dog. Pip is 20 inches so needs a treadmill that has about a 40″ run area to trot comfortably and have some spare at either end. If I wanted to run her hard, I’d need a large machine but I use treadmills for walking and steady trotting, not running.
With your dog lying on one side, take a tape measure and, gently swinging a front leg forward and a rear leg back, measure that length and add some extra at either end. This sounds easier than it will be for many of you which is why I suggest the back measurement first. By this equation, Pip needs a 48 inches run area (42″ with her legs gently extended + 6″ for wiggle room).
So, you can see that this is not an exact science. If your dog takes long strides, go with the biggest number you get. If your dog is takes shorter strides, as Pip does, you can probably err toward the shorter end of your scale. And remember, the faster you plan to go, the longer your dog’s stride will be.
Now you know!