So how small is Dex's dick? The Rock was often jabbing at Deckard's size, but there's one exchange from the movie that confirms it's more than just shit talk.
When the Dex, Hobbs, and Vanessa Kirby make it back to Shaw's Lair, Hobbs sees all of Dex's sports cars and tells him he's overcompensating, obviously for his small penis. Shaw retorts that, if anything he's UNDERcompensating. This suggests that his dick is SO small, that his fleet of McLarens don't compensate for just how small it is.
So what are we thinking? Two inches?