Is there any connection between penis size and whether a man wears boxers or briefs?
There is a myth that men who wear boxers have larger penises. And you could speculate that men with larger penises might choose one or other for comfort reasons. There was apparently a study done to see whether this was true. The results showed no correlation between penis size and underwear. So, there appears to be no connection, regardless of whether it would be cause or effect.
There are a few possible sources for this myth.
First, some believe that if you wear boxers your non-erect penis and scrotum will hang more loosely and thereby appear longer when you're naked. This is certainly possible.
Second, there is evidence that men who wear boxers may be more potent, i.e. they may be more fertile. This is because briefs hold your testicles closer to your warm body and the body heat is bad for sperm.
No, there is no connection, merely a comfort factor - some guys prefer support, where others prefer to be loose.
Size (when erect) does not even come into play, as the flacid penis size does not indicate the size when erect. Men can have very large penises but be quite small when flacid, likewise, some might be larger than many when flaccid, but not grow much in size when erect. They are all different.