Here is the actual one that I used and there is a sizing chart at the bottom of the page. I used the X-small and these should come with an insert to adjust the cage as they grow for potty training purposes.
Bargain Hound® Wire Crate - New Puppy Center - Dog - PetSmart
X-Small
(18")18"L x 12"W x 14"HUp to 12 lbs.Pug, Yorkshire Terrier, Chihuahua, Shih Tzu,
Pomeranian, Maltese and similar-sized dogsSmall
(24")24"L x 18"W x 19"HUp to 25 lbs.Border Terrier, Miniature Dachshund, Miniature
Poodle, Silky Terrier and similar-sized dogs
If your yorkshire terrier is say only 7 pounds when he is an adult then you would only need the X-small. You can probably tell their size from the parents. Then once they are trained you can just take the insert out. There will still be enough room in there for them. Hope this helps.