There are varying degrees of a shy bladder. Some guys just can't use urinals if there are people around, others can, but there have to be dividers. Sometimes if it's a familiar place it's fine--like the work restroom. Usually its bothersome if the guy can't go unless the restroom is empty, or and "individual" one, that locks, it's not that big of a deal.
Penis size doesn't have anything to do with it. Guys can be small average or large. It's usually about getting over the perception that people are watching or would be wondering if you are peeing or not.
This gets in the way the most if the guy is exposed to situations where it's inconvenient, like football games, bars, or situations where multiple guys have to go at once and it's hard for the guy to go. Anyone that has this can google it, and look at online sites. A lot of sites say you need to break the problem with having a "pee buddy" where you go to a whole bunch of places and pee with someone else who doesn't have the issue. You start out by peeing alone in the bathroom, and work your way up to the "trough" bathroom. I think that's a great way to break the cycle of going to a urinal and not being able to take a piss, but you have to talk to a buddy about the issue, and then set aside a fair amount of time to go through the motions of peeing together in public. Websites on the subject can give some step by step things to try yourself.