There are a few things you need to take into consideration when measuring a dog for a wheelchair. The first is the height of the dog. You will need to measure from the ground to the top of the shoulders. The second is the length of the back. You will need to measure from the base of the neck to the base of the tail. The third is the width of the chest. You will need to measure from one front leg to the other at the widest point. Once you have these measurements, you can then begin shopping for a wheelchair that is appropriate for your dog's size and weight.