I think it really depends on what you are looking for...but remember just because you plan it doesn't mean it will happen. My situation is a bit long, but in short, I wanted a bigger yorkie. I knew I was getting Suri from my breeder and when i was ready and she had a litter we picked the larger of the two.We thought she would would mature to about 4.5 pounds, and at almost 6 months she isn't touching 2 pounds yet. Her sister ended up being the bigger of the two girls.
I love her no matter what size she is, it's just funny that for most it is the other way around.
make sure that your pup and their parents have necessary tests done to decrease the chance of you getting a sick pup, especially important as the smaller pups tend to have more problems. (but of course any of them can) good luck on your puppy search...how exciting!