Being the smallest of the litter (there really isn't a "runt") doesn't mean she will be the smallest in 3 weeks. I've seen my smallest become my biggest and my biggest become my smallest.
Alot of time at birth it just depends the position in the uterus. Then the more aggressive eaters may gain more...etc... 5 weeks is really too young to get an accurate adult weight.
I'd ask questions about the parents past litters, the grandparents....sizes, lines...
Oh and remember...regardless of what you read (LOL and there is alot of not true stuff out there) just because a puppy is small...doesn't mean it is automatically unhealthy. It does however mean you need to be more careful with them and make sure they eat well and other precautions. I hate reading people think just because a baby is small its sick...LOL.
good luck in your search. I think you are asking all the right questions and doing everything correct
