The 15-18 would be the best for an 8 week old puppy, but is still probably going to be a bit large. The 18-26 would be the right size for a full grown GSD.
Since pups grow so fast, I like to get the fully adjustable nylon collars. The ones with the slide buckle instead of regular tongue buckle with holes, that can be fully adjusted by several inches. Like these:
Then when the pup has grown to more or less full size, I'll get their "big dog" collar.
Since pups grow so fast, I like to get the fully adjustable nylon collars. The ones with the slide buckle instead of regular tongue buckle with holes, that can be fully adjusted by several inches. Like these:

Then when the pup has grown to more or less full size, I'll get their "big dog" collar.
