I think it depends more on the individual dog and its build and natural athletic ability than on size. I have one 65 lb female who is extremely agile and quick, like a malinois, and one just a little smaller who is not a very graceful dog. And I have one 50-ish lb girl who's pretty athletic but nothing spectacular, which goes against what a lot of people say - that smaller dogs are more athletic. I've seen dogs of every size and shape and you can't always tell just by looking at them.
I've heard of JRT's doing schutzhund so I don't think there is such a thing as too small for a GSD.
I like to get puppies and raise them to adults to work, and when they're little you never know exactly how big they'll end up being. But if I was going to buy an adult, as long as it was within the breed standard, I'd be more concerned with how the dog works than with its actual size.