dog question

My friends have a puppy who's about 16 weeks old and weighs 5.5 lb. He's a mutt who looks a bit like a chihuahua and a bit like a shiba inu, and he's gained about half a pound in the 3-4 weeks they've had him. I have very little experience with puppies that young, and none at all with toy dogs. Does anyone have a guess at how big he's likely to get?