I agree with Anna. It's hard for a breeder to guarantee size, but she lied to you about his weight. Did you get to see the parents, or do you know what size they are? Of course, she may have lied to you about that too if you didn't see them.
I have a smaller yorkie and there are times I wish he'd get bigger, as adorable as he is.

He's a tough little guy, but we really have to be careful around him. They're so FAST....lol.....you'll see
Congratulations and good luck with your new baby! Would love to see pics...we love pics around here!