you will get to know just looking at them when they are the right size, if you put the ring on the gently pull it back to make sure it can't slip off over the toes.
You say it wouldn't go over the knee joint, it shouldn't need to, put the ring over the 3 forward toes then when you pull the ring on the back toe will apear which you should be able to just pull through or if a bit tight use a match stick, quill or blunt needle and push between back toe and leg under ring and it will give you a lever to pull the back toe through with.
Phil