Rear Wheel Nut Size

Hello, just wondering if anyone knew the size of the nut on the rear wheel of an America 2010, I measured the nut with a digital micrometre and it is telling me its just under 24mm so I purchased a 24mm long reach socket ( Silverline ) which came today but it feels pretty loose to me. Could it be the socket is a "big" 24mm or is the nut 23mm. I have a Haynes but I can't find the nut size in there ( and that's even with my readers on ! )