This is my first AR15 and I'm looking for ammo.

What is the best grain size and why for a 16 inch, 6 groove, 1x9 twist barrel?  The common sizes seem to be 62grn and 55grn.  

Read the Ammo FAQ above.  The consensus seems to be M193 55 grain FMJ ammo is the best general purpose round.  I personally like both M193 and the M855 and/or SS109 ammo.  

I agree that the mil-spec 55-gr ammo is the best all around stuff, especially for a 14.5-to-16 inch barrel AR15. I would rate Win Q3131A, Lake City XM193 and the South African M1A3 surplus as my personal favorites. The 55-gr gets maximum velocity and frag potential in the short barrels, still netting 2700fps+ past 100-yards to ensure fragmentation of the bullet and max wound potential. The heavier 62-gr bullets will quickly fall to less than 2700 fps after leaving the barrel and will perform a thru-and-thru in one piece, not dumping their full energy into the target. Read this site's excellent ammo FAQ to get more info.
Tight Groups,
phoenix 27