How big is my penis? Which method of measuring is best?

I recently say that you're apparently supposed to measure your penis while laying down, from the base to top with measuring tape or w/e. I don't have any so I used a string, did the same thing, then cut and measured the string. Usually when I measure the side of my penis with a ruler it's 6 inches, but this string is saying 7. Did I do something wrong? I don't understand how there's such a big change in length..and which is right??