The TuffStuff lead has a metal ring as the stopper, you can see it here on the right of the picture:
So, whilst there is adjustment (the black slider you and see on the left of the loop), it is set so that there is an absolute minimum loop size. Then there's a sliding stopper which you can use to stop it opening too much if you want (although I got told off by
@bird for using this - in the nicest possible way, of course

).
I have another limited slip which has a sliding metal thing which you position so it could be any size. The benefit of the fixed ring on the TuffStuff one is that it can't slip, however much the dog is pulling. And, since the TuffStuff one is a nice wide collar, I would be happier using that with a dog that might pull during training than I would be using my other limited slip, which is a round rolled nylon one.