Lol, personally, I would love to see a 50lb Yorkie, I think it would be gorgeous, but I wouldn't want to do it's hair, that would be a lot of upkeep! Here's a growth chart, it's just estimation and is listed in ounces.
http://www.yorkietalk.com/forums/growth_chart.doc They do have growth spurts, so you never know. How big were his folks?