A friend of mine has gained 1.5" BPEL and .5" MSEG with nothing more than just jelqing and stretching (warmup/ warmdown, kegels too). It took him about 2 years so far.
You can always keep jelqing--you may eventually get to the point where you need to add more things, but I know guys that have jelqed their way to some massive gains (they used other exercises as well though, keep that in mind).
There are probably limits on what a person can gain, but PE is not hard science, and a lot of opinions in this area might be speculation. Based on my observations it appears most people quit long before they maximize their potential. I.E. Often guys reach a size they are happy with and decide to quit. That probably throws off the equation if someone tries to research what the maximum possible gains are.
About skin: it can be a limiting factor, albeit a very minor one. As Big Al said, the only issue with skin is getting too much--skin is one of the easiest penile structures to deform and manipulate. Its really the tunica, fascia, and ligaments that are hard to deform and grow. The tunica albuginea is probably the most difficult penile structure to deform.