This whole "stretching" idea is a joke. Condoms do stretch lengthwise, but only with a huge force pulling it down (as in the broomstick example cited above). When you put a condom on your erection are you going to have hundreds of pounds of downward force to ensure it stays "stretched?" Of course not.
Condoms don't actually stretch lengthwise when unrolled onto a penis. If you're too long, the condom simply wont cover your entire penis. This, of course, is where larger-size condoms come into play. And no, it's not just for male ego. Any STD prevention manual will advise that you "unroll the condom so that your penis is COMPLETELY covered." For a regular condom, "too long" is greater than 7 inches.
Girth can also be a problem with condoms, although most condoms actually DO stretch girthwise--they have no choice but to (try and visualize it in your mind). However, even though a standard condom will accomodate the girth of a coke-can, it won't do so comfortably. Here again is where large condoms come into play.
I personally use Magnum, mostly because of my length. Although my penis is thick enough to meet the "toilet-paper-roll" criteria, and therefore technically mandate a large condom based on girth, length I found is the biggest complication. For the record, I'm 8.5 inches.
Hope that helps.