My first question would be how does your Mom define "small German Shepherd"? Your Mom might have in mind a size that is just not possible in the breed. Or she may have an image of an over-sized dog to begin with, and what she considers small is actually correct for the breed.
I don't think being the runt of the litter will accurately preview adult size. The size of the dogs in the bloodline is a better gauge of adult size, in my experience. If you want a dog that is on the smaller side of medium, look for bloodlines that routinely produce that size. Not just the parent dogs, but going back to grand-parents, etc. I think that will be a more reliable indicator of adult size.