Funny you mention that -- I was just looking at what I believe is the current issue of Men's Fitness in the gym today. Vin Diesel is on the cover and I was looking at it thinking how ill-proportioned and lame he looks. Have a look for yourself (remove the spaces): www . shoppingblog . com / 2011pics/vin_diesel_mens_fitness_june_2011 . jpg
Hardly any vascularity, even though he's clearly flexing, biceps get lost somewhere between his forearms and shoulders, underdeveloped triceps, wide-looking waist compared to his back, shoulders overpower traps.
All in all not what I would call the epitome of "Men's Fitness." As for the arm size, heck if I know. He looks short b/c of his proportions, but I'd say 17 could be right.