This is extremely subjective and very opinion based.
My personal opinion is, that penis size can effect your overall success in life. But, this also depends on where you are. It also depends on your overall opinion of your self worth.
I can only speak for the USA. We have saturated the media with what the expectations for men are. I can't speak for other countries because I haven't left mine. But our culture revolves around sex sells. Women want size. Men want size. Does this apply to all, no, but it applies to what the media imposes upon us.
I am not sure if there are statistics, but I have a gift for observation.
Penis size effects confidence. This does not apply to everyone, but, think about Big Dick Energy. I have a coworker who has confessed he is pretty hung. He has a strut and confidence and is able to move through life with ease. He's a nice guy with little arrogance. He has ambition and I know he will succeed in his endeavors. I have met men who have average (statistical average 5.5" roughly) and some under average that over compensate. They create an exaggerated image of who they would rather be instead of embracing who they truly are.
I can speak from personal experience. I am half Korean. I am just above statistical average. I have HORRIBLE self esteem. I have little to no confidence because I grew up closeted and exposed myself to whatever gay culture I could. Back then when I was discovering myself, most of my outlets were gay porn. It created a warped sense of what was desired over what was realistic. I grew up thinking I would never be adequate for a future partner because I didn't match what was in the media. I eventually learned to do my research and listen to fact over fantasy... But the damage was done. I've spent the last 5 years working to boost my confidence. I have self image issues and little confidence in my success. I can say with confidence that I have skills, and natural skills that make me successful, but because of my poor self esteem, I cannot achieve the success I have the potential for. I am working to change this. But, I grew up thinking that, because I didn't have noticeable muscle, or a large member, that I would not be desired.
Basically, my success was hindered because I grew up with a warped sense of reality. It impacted my confidence which effected my success.
Now, I am not saying that all the successful men have large dicks. But those who are successful, but humble and confident, exude big dick energy and typically possess one. There are men who are successful, because they recognized their talents and because confident in that instead of using a personality to achieve success.
This subject is a never ending circle. Without sufficient research, this is all a matter of observation and opinion.