I am not sure about skin stretching, but for increasing testicle size I would try these.
The ball zinger. I used it, and it increased my ball size big time.
Damiana - I also used it and many others on the board will say that it increases testicle size.
Other herbs you might want to try are ginseng, cordyceps, horny goat weed, and tongkat ali. I have seen a small amount of info on these increasing testicle size.
Ball pumping might help. I have just got a special large pump to try it so I can let you know if it works or not.