Buy the large size with easy tear-open package. This has nothing to do with your penis size. It has to do with the ability to slip it on quickly. Large size condoms slip on easily, and therefore are most likely to be used. A struggle to put on a condom will result in loss of erection.
Condoms only go on one way.
No one has ever told me this fact, and I have yet to see this fact stated on the packaging. Practice with a condom. You can easily determine which way it rolls by starting the roll down your thumbs. If it doesn't roll flip it over.