I'm guessing it's probably true. Kind of*
But not by a whole inch in terms of erect size I would think ... ?
I've had many opportunities to see penises in both soft and hard states, and I think they are on average slightly larger among the population who define themselves as out gay men.
*But, there's a few things to remember.
1. Some of the "bravado" that you need to actually go ahead and define yourself as gay, when there's really 20 or 30% of people, or whatever it is, who are functionally "bi" ... that might be connected with dick size.
2. In some ways, maybe even interest/fascination in penises may be connected with homosexuality. Not the sole "cause", obviously. And I'm not saying any or all of the straight men who use this site "must be" gay. Not at all, I definitely think that would be nonsense. But, however people's sexualities are laid down, some kind of interest in the penis will surely have some kind of relation with homosexuality - arguably, it's what homosexuality is. And if your own is a bit bigger, you are more likely to continue to take an interest ... maybe ?