I dunno, if having a big penis was the key to happiness one would think there'd be no big guys on this site at all--we'd all be too busy frolicking in the sun and having non-stop sex and laughing joyously at our good fortune as flower petals rained around us. Having a big penis is a key to getting attention for having a big penis, and that's about it. And that can be fun and nice and flattering, but really no more than that. Attention and flattery don't bring happiness.
So at the risk of sounding tautological, I would posit that the key to happiness is simply to be happy. If you base your happiness on conditions, circumstances or qualities, then what happens when those conditions, circumstances or qualities change or never arrive? You're left with a handful of dust. Looks fade, dicks shrink, money gets lost on one bad spin of the wheel. But If you wish to be happy--truly, deeply happy--then learn to accept yourself and your life, unconditionally, as you are, as it is, right here, right now. Which doesn't mean you can't work to change things or strive for goals, or that you have to have some sort of naive Panglossian viewpoint that this is the best of all possible worlds. Rather it's to realize you can be happy even if everything in your life kinda sucks--and often times it will. Again, that doesn't mean you have to be satisfied with any situation--just that you can choose to not spend your life shaking your fist at an uncaring sky or staring dolefully at your own crotch thinking that you're three quarters of an inch away from true bliss. Because happiness is not something out there somewhere, or hidden in our pants, or kept from us by "If only's" or "If then's." Happiness is not a reaction to having or doing anything. It's not a cause and effect situation. Rather it's an active, experiential way of being. It is the freedom in any moment to choose courage, to choose resilience, to choose to be who you are and nobody else. And, as such, is all of our birthrights--whether one has three inches or thirteen or no inches at all.
tl;dr: happiness is not measured in inches or centimeters.