X-Small is for up to 8 inches - Silky Terrier, Boston Terrier, Maltese, Pug, Jack Russell Terrier 100% Cotton Jersey Machine Washable for easy Care Rhinestone Accents Sizing: Measure length of pet's back from the base of the neck to the base of tail. If the measurement is between sizes or if pet has a stouter build, we recommend selecting the next size up for a proper fit.