Each harness chosen (especially on the Internet), you specify the measures, you only have to take the measures of your dog to find the ideal size. However, I advise you to choose a harness with as many adjustment points as possible. But to make sure you choose the right size, measure your dog's chest size (the size of his belly, just behind his front legs). To give you an idea, depending on your dog's breed and size, here are the "standard" sizes for the most "popular" breeds