The answer to this question really depends on your dog. Some dogs fit more easily into coat sizes than others. If you want to order online but are worried about finding the right fit for your dog, make sure that the company you are ordering from does easy returns, and order the same dog coat in multiple sizes. Double checking that an online order can be returned is a good practice in general. It is of course always better to try something on before buying it. Therefore, the conclusion is that you should have your pet try coats on whenever possible, but it is not the end of the world if this does not happen.