I see AKC has done quite a revamp of their whole website since I was there just about a week ago. They used to state just above the breed standard 4 to 7lbs. On this page that I found it is in an area called "Right Breed for You?" just above the standard.
Yorkshire Terrier Page
I haven't really looked around at what else they have changed on the site. Their previous site had a lot of different info areas so who knows what and where everything else is at this point!