What size is the nut on the engine drive puley

2007 1200 custom Sporty 5000 miles on the clock.No problem getting gears and selecting neutral all that works great.But yesterday my green dash neutral light stopped working.Now I know that with me it will not be the dash bulb.Bound to be the switch...So today I removed the 1/4 rt hand side side engine casing,Now I see that I have to remove the drive pulley to gain access to the neutral switch.I am now looking at a big nut with 2 2 small allen bolts...So the question what size is that big nut so I can go and buy a socket/spanner/wrench.Any help.