My smallest (Bailey) is 2 lbs 2.5 oz and is just shy of 8 months old. He's going to be neutered this Thursday so I'm pretty sure he'll gain a little after that.
My largest (Ellie Mae) is 5 lbs 15 oz and just had a litter of 5 puppies 2 weeks ago. Those 5 are really taking it out of her....I'm going to keep her on the puppy food for a while longer, even after the puppies are weaned.
Now my Cairn terrier is almost 20 lbs...but I don't guess she counts even though she's just 4 months old.
