Looking at my buddy Jasper (Aussie Collie Mix), and the pics of Aussie dogs in their shoes below, I went with the 2.5inch paw size shoes. Jasper had a miserable time in them! They kept sliding down his very soft fur, and I can't figure out if it's because of his soft fur, or because they're too big. He was so mad at me for putting him in them, he licked his paws for a few hours afterwards while giving me the evil eye. He wiped out pretty hard in them while trying to jump, and now I'm hoping he's not actually injured.
Moral of the story: Don't assume your dog's paw size from the type of dog, like I did. Also, trust the sizing chart, and test it out with your dog to see if they can handle it.