You wouldn't buy clothes for yourself in the wrong size so you are going to need to make sure that you know your dog's correct size before buying him clothes. Getting those sizes is as easy as measuring your dog. The girth is straight down his back, from the bottom of his neck to the beginning of where his tail starts. You will want the circumference measurement of around his neck and the width of chest as well. With these sizes, you will be able to find perfect fitting small dog clothes.
Sometimes you will find small dog clothes that have long sides, especially in the cases of cloaks and dresses. It is important that you make sure these kinds of outfits fit your dog without dragging the floor because this will get to be an aggravation for your dog when trying to walk.