Everything I have read says that them reaching full size can vary. Alot of time they will reach their adult size at around 6 months or so, but that is not true for all dogs. My Abigail grew up until she was a year old, where Summer stopped growing probably around 6 months or so. There is a chart on here that can estimate the adult weight of your yorkie. It worked right on for Summer, but not Abigail because she is a little larger.
I never kept my babies on leads when we are outdoors unless we are in a public place and not at home. They have always done well. Lola on the other hand has to be kept on a leash because she likes to run. I got her when she was a little over a year old. She is slowly getting to the point where she doesnt have to be kept on a leash.
As far as the coat question I can't help, I keep my girls in full coats. Good luck!!
