Either 3 or 8 MOA are okay for turkeys.  The 8 MOA can be used as "rangefinder"  on a turkey's head.  At 50 yards, 8 MOA covers a circle 4" in diameter.  If the "dot" fits inside his head, he's inside 40 yards for sure.  My dots are 3, 6.5, 7.5 and 8--Aimpoint, Trijicon, Doctor and FFIII.