Need to find out what size this nut is........

I'm having problems with getting one of the bolt & nuts off of the rear knuckle, The one I'm having problems getting off on the knuckle is the bolt & nut on the side where the rear shock mounts. I'm sure the bolt head is an 18mm, but what is the nut size? 7/8" is a little to big... so what is the mm size below that... 21, 22mm?

I have actually bent my 18mm wrench pretty good trying to get this darn thing off. But I don't have the correct size wrench or socket to take the nut loose. The pass. side was no prob. on all 3....

Anyone?

Thanks!
Jeff B.

it's a 21mm ..I bought a 16,18.and a 21mm wrenchs and sockets ...16mm was for front swaybar links ..18 mm was to take drive belt off with and rearcontrol arms along with the 21mm... and had to use my hydrolic jack ..to get enough torque ..to break mine free .......they seize in the bushing ....dave