Depends on the behavior of your GSD, somewhat. Well behaved, doesn't lunge or pull, not prone to go after other beings...then a 5/7 oz single ply with the buckle end turned under for 2" to keep the D-ring or square loop in place. I always glue, rivet and stitch for big dawgs. I'm picky about finishes and stuff like that. I use Olive Oil almost exclusively. I try to keep customers wanting black. That way I can do the vinegaroon and cut back on chemicals and ruboff.
If your GSD is all of the above, and I have a couple, then I would recommend a 1-1/4 or 1-1/2" (I've gone up to 2") wide 2-ply collar. I usually go with a 5/7 oz doubled. The wider collars prevent or diminish the possibility of larynx damage.
If your GSD is on it's last legs and has trouble keeping up with you, a 3/4" collar will work. Otherwise, a minimum of 1" wide.
Most of my work now goes to competition dogs - Schutzhund, Personal Protection, K-9, etc. I do collars up to 3" wide for the Bully breeds, corsos, rotties,etc.
Now, I'm not sure, but there may be some arguments about this but it's my way and I'm comfortable with it. I will NOT use chrome-tanned leather on a dog. I used to haul hazardous waste on the west coast for years and I hauled plenty of chromium salts from the old tanneries to a place in Idaho that keeps them buried in 55 gal. drums for future generations to figure out what to do with them. All one has to do is do a Google for the MSDS on chromium salts and it's double ugly. I spend a few minutes more with the leather and it's just as soft. Plus, the chrome-tanning process destroys the cell structure of the leather. That signifies significant weakening to me. It might work for awhile on a chihuahua, but it's an escape waiting to happen on a big dawg.
g'luck and have fun.