Socket Size for Kohler Simplice Faucet Remote Valve Stud Nut

The remote valve for our Kohler Simplice faucet has rotated out of position and I'd like to tighten it down to the granite counter top. The instructions show the long nut being tightened with an adjustable wrench.
Now that the disposer, deep sink, and pull-out trash bins are in place and with my recently injured knee, I need an easier way to tighten that nut. Looks like a deep socket with some extensions would do the trick, but I don't know the socket size. Anyone know?

Thanks!

It's 10mm. Took out the trash pull-out and I could see what I was doing.