Are you hesitating between two sizes? You do not know where to start ?
First of all, rely on this photo to measure your dog in the right place.
The back length is important, it helps determine if the garment will be long enough to cover your dog's back. The chest circumference allows to know if it will be able to close well and be comfortable in its clothing. You should count an inch or two (or cm) of the coat to leave room for your dog for his coat or if he is gaining weight.
It is important to note that each model of clothing may vary depending on the company. If your dog takes XS in one model, it may very well be that he takes SMALL in another. You should rely on your dog's measurements and compare them with each new garment.
If your dog is between two sizes, it is better to take a size above. Your dog will be more comfortable in clothing that is a little larger than too small.