Angie,

i have a medium dog. He's a GSD, but kind of small (he was the runt after all ...lol) he weighs 50 / 55 lbs. i've seen these leashes for small, medium, and large dogs. at first, i used the thickest, but had to take it back, it was just too heavy for the both of us, and also, way exagerated, as i was getting him under control. so, i went down one step and used the medium chain. (which we also used for our rat terrier who is 20lbs.)

i've seen other thicknesses and lengths, but i don't know where people get them from. these leashes that Kmart sells are 5ft in length.
(I have to admit, they are often under stocked, so finding just what you are looking for is based on luck.)