I personally prefer for mine to be 4 to 6 lbs.Mine are a bit small.Beatrice is barely over 3 lbs and Sebastien is 3.7lbs...although he is only 11 months old.My hope is for him to reach 4.25 lbs.
Knowing what I know now, if I was searching for a yorkie I would look for one that would mature around 5 to 5.5lbs. I find we have to be so careful with our "tinies" Beatrice jumped off the couch a year ago and broke her leg.They can't go down the stairs on their own,and when it's cold they don't go outside.
However,I would not trade my precious babies for anything.I love them just the way they are