> condoms, which have a real-
> world failure rate of about
> 18 percent
> With consistent and correct
> use, condoms have a failure
> rate of 2 percent. The
> typical use effectiveness
> rate is about 18 percent
And one of the most common issues buying the largest size available even when you don't have a 12" as thick as drink can tool.
While it's been 16-17 years since I had a sexed class i still remember the girl who came and told us to buy regular or even ("small") since it was still when I was 14-15 then she took a the smallest size Durex and rolled it over a 2 liter soda bottle and said unless you are bigger don't buy magnum super XL.
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.
In other places it might be different I know people think that Asian thing is a trope but it's not Durex reduces the width from 54 to 49mm for the Asian market which is a bitch.
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.
Abstinence has a "consistent and correct" usage rate of 100% (within rounding error). Of course, in the real world, abstinence has a pretty poor rate of preventing unintentional pregnancies, when not combined with other methods.
Real-world use encompasses everything from people keeping condoms in their wallet (exposing them to heat + friction) to people deciding not to use a condom (whether because they forget, because they believe the other person is on birth control, because they decide to just "take a risk this once", or anything else).
I don't think that counts as part of the "use effectiveness rate", which AFAIK are only those who (claimed to?) have used one but failed.
It actually (usually) is, because that's a really important part of the effectiveness of a method. That's why I added the part about abstinence - it might work if you're consistent and committed about it, but people still decide not to use it as a method.
If someone invented a condom that had a 100% perfect-use effectiveness rate, but it also made sex uncomfortable to the point that people decide not to use it most of the time even when it's available, it's real-world effectiveness would be low. And that's the important number to measure.
I'm a "grower". The size difference is so significant that any condom that still feels comfortable when erect will easily slip off when flaccid. Smaller condoms are actually painful because of the ring at the base.