First off I must say that no yorkie is perfect. They all have their faults. In the show ring you usually see yorkies of all sizes. You will see a few small ones that are under 5 lbs and you will see a few that are over 7 lbs and many in between. The height at the shoulder will vary depending on body build of the yorkie. A 7 lb yorkie with a short cobby body might only be 8 or 9 inches at the shoulder where another 7 lb yorkie that is lean and leggy might be 11 or 12 inches at the shoulder.
A square body is desired but sometimes, especially with bitches, you will see some that are a little longer in body. There is no set rule as to what is to long. You have to look at the overall structure and coat of each individual dog.
Here is a link to a interesting test and discussion on evaluating dogs.
CFYTC About Yorkies - You be the Judge
Judging yorkies can be very difficult and it is often a matter of beauty being in the eye of the beholder.