It doesn't matter if you had a 10 inch penis, wearing tighty-whities is not going to affect it. The thing you need to worry about are your testicles. If you have something clamping down on your testicles all the time, bad things can happen, so that's what is truly important -- the size of your balls, not your shaft.
Tighty-whities and tight jeans do irritate me, so what I usually wear are boxer briefs, because they don't scrunch up like boxers, but don't suffocate me like briefs do.