Kong squeakair tennis balls

Durable squeaky tennis balls in multiple sizes.

XS is for tiny dogs like Chihuahuas and Yorkshire Terriers (and cats!)

S for Lhasas, Small terriers, 

Medium is standard tennis ball size suitable for Cocker Spaniels, Border Collies, JRT, 

Large for large Collies, Labrador, Golden Retrivers etc

XL for large breeds

If in doubt size up, especially if your dog is a catcher!