Natalie, a member here has a female who's 1 lb 5 oz (full grown) and there's the chihuahua Tia owned by another member here who full grown is 1 lb 3 oz so it would have to be smaller than both of those. There's a Munchkin at Swankpets but she weighs 1 1/2 pounds so, obviously not the smallest.
I know they don't just go by weight though. There's also this:
http://utubelaughter.blogspot.com/20...iving-dog.html