That depends, does it still count if your piece looks more like a misshapen blob of flesh than a human penis? If so, then the skies basically the limit. From silicon injections, to fat grafts, to abusive amounts of pumping, I think inhuman girths can be achieved. Even in the case of pumping, dudes like Johnny Rasputin/Castro/Julio made massive gains if you look ate their before/after pics. Natural methods like jelqing I think give some results that are both more moderate and natural (think 0.25-.75 inches). There are also things like "clamping" that involve restricting blood flow for a period of time to make gains. This is of course a major source of potential gains, as it resembles things like priapism that have given some pornstars like Shorty Mac their monstrous girths. The one caveat here is that such a thing may result in a floppier or less natural looking erection.
I've heard the same. Gains are essentially for life.
I suspect you're right. This would explain the discrepancy
@JGUIDO is talking about. Cock size seems to level off at somewhere a bit below 9 inches in length and 6.5 in girth, give or take a bit.
How big do you think James Deen is?
I believe this dude posted ITT way back. IIRC he was about 7.5.