Both of my blowers are 78" wide and the auger chain is #60 size roller chain... I see #60 used on a lot of 60" blowers for the auger, too.  If you are driving the fan via chain, #60 would be plenty strong.. but you should still use an appropriate smaller size shear bolt/pin as if you were driving it via a PTO shaft.  This would be to protect the tractor gears.