Sorry it took me a while to make it back here. My Sheltie wears a small Sleepypod Sport Clickit and weighs 11.3 kg (24.9 lbs) (I'm pretty sure he was a different size when measured for the Sleepypod Utility Clickit). He is about 16.5" at the shoulders with a girth of 22.5" and neck circumference of 14". According to 3 vets his weight is categorized as "ideal body condition". I mention this because I would probably have to go up a size if he was very overweight. My pup tends to be between sizes in collars and clothing and he is right on the upper end of the small size. One thing I have noticed is that the straps loosen up a when the dog is laying down vs. sitting or standing. I also have to tighten it up in the summer when his undercoat isn't as thick.
When adjusting the harness keep in mind that it should be snug like a 5 point restraint for infant car seats. It may not be very comfortable but it is much safer.
Hope that helps!