How to judge how wide your dog's wheelchair will be.
Since every dog is unique, their wheelchair will be adjusted to best fit them. Meaning the wheelchair width will vary depending on the size of your dog and the size wheelchair they need.
The best way to judge how wide your dog's wheelchair will be is to measure the width of their body (at the widest point) and then add six inches to either side.
Width of Your Dog + 6" on either side = approximate width of wheelchair
Additional Resources:
5 Things You Must Know Before Buying a Dog Wheelchair