I wouldn't expect any health differences with the smallest pup in a litter. How much smaller is she (he?) than the rest of the litter? Puppies don't all grow at the same rate, and when that young can put on several pounds a week, easily 10-12 pounds in a month.
Temperament wise, the breeder can probably tell you what they've observed so far. Halo was small, although I have no idea if she was THE smallest. She was only around 11 pounds at 9 weeks old, I saw it on her health certificate from the vet. We got her a week later and she was 14 pounds. Her adult weight was 54/55 pounds, just under the mid-point of the standard, but much smaller than our previous females. She was the most confident dog we've ever had, everywhere she went it was like she owned the joint, and she was one of the most bombproof dogs on our flyball team. Nothing, absolutely nothing phased her, not even when she was charged by a dog at practice (we practiced at a shelter, the dog was being walked by a volunteer and it got away from her) and it latched onto her shoulder. A teammate had to run over and get the dog off Halo while I held onto her so it didn't escalate into a full on fight. Halo was PISSED, she wanted to kill that dog. And when they got the dog back inside the building, she shook it off like it was nothing and continued doing the run. Everyone was all concerned about how it would affect her, "is she okay?" and Halo was like "can we just play flyball now???".

People would ask if she was sensitive, I guess a lot of GSDs are? I'd always reply "no, she's INsensitive", lol.
Cava WAS the smallest in her litter. We joke about her being the runt, but at 14 months old she's almost 60 pounds. Her brother and sister are at least 15 pounds more than she is and were bigger at every phase of development. She was described by the breeder as having spunk, and she definitely does. She doesn't have the in your face kind of confidence that Halo had, but she's never been phased by anything either, she takes everything in stride, and she's very fast and extremely agile.