There's no way to guarantee size, but as it's already been said, you need to find someone who knows their lines well and knows what those particular parents throw and what has been thrown by their parents and grandparents.
And yes, it can definitely happen w/2 seven lb parents. My Brooke who just had babies is 6.5 lbs, and the daddy is 4.5. He comes from a line that throws smaller pups, and so does she. My little Sophie was out of their last litter and was one of the bigger puppies, and she's 3.5 lbs at almost a year. That particular litter had puppies that are now: 2 lbs, 2.5 lbs, 3 lbs, 3.5 lbs, and 4 lbs. Also, a good breeder who keeps their pups (ESPECIALLY if they're smaller) until 12-14 week or MORE will be able to tell you more about the possible size then...notice I say "possible"...because there are never any guarantees, but good breeders have a better idea.
