Plus just rolling it over a 2 liter bottle doesn't account for the physical side of what's going on. You can stretch a condom pretty far, but that does not say anything about how it stands up to friction. If a condom is too tight, it will break on you.

I hate talking about this because people just assume I'm lying, but... I've had normal sized condoms break on me a couple times, it's frightening. Magnums do not break on me, and fit much better, so that's what I use (the XLs are too big). I just don't fit safely in a normal sized condom, it doesn't matter one bit that I can fit them over a bottle.

I know I had issue finding a good size in Hong Kong before and I'm not adult film star endowed by any stretch of the imagination.

That said in the west you are likely not going to be needing magnum sizes either.

The regular size is well within average girth and we'll over the average length the regular size for most brands is 18-19cm from the base ring to the tip(there are ISO standards for Condoms :)), unless you are measuring yourself from your anus you aren't likely to need anything longer than that.