I'd say it depends on the movie... Some of these movies, the minimum is 9" it seems. Others have very average guys and at 6 1/2" (NBPenis EnlargementL) you might be one of the larger ones. So if you only want to do one-the question would be what kind of movie, what director, etc. Now if you want to be big enough by "adult entertainment standards" in a general way, then I'd say 7.5" NBPenis EnlargementL minimum. And if you have that, then 5.25" MSEG would be fine.
Source: Major fan of porno