I have worked with the finest furry tailors in the world, and am here to share my bounty of knowledge. Here's how to measure your pet...
Measure from the base of the tail (where it connects to the hindquarters) up to where the collar typically sits at the base of the neck, then follow up with a generous back scratch.
This is measured by giving your pet a hug, during which you measure the entire circumference (all the way around) at the widest point of your pet's chest. This is typically right behind the front legs.
First, plenty of kisses. Then, measure the neck's circumference (the distance around it) at the point where the collar sits.