XXS= my 5.5 lb deer head wears the XXS and it is has room on neck and chest to be made smaller for your dog, Mine has 9 inch neck 13 inch chest and XXS with room to adjust up or down 2 inches. This is well made and fits her proportions nicely. Honey is bald underneath and this harness isn't irritating/rubbing her skin like many other styles that just don't fit her slim neck, barrels chest and long skinny body. I am very pleased with the purchase.