Its 1 7/8" .
Don't get the adjustable packing wrench.
They suck a$$. The nice wrenches are $$$, but are worth their weight in gold. The lock nut on the packing setup is often TIGHT, you need to have the right tools.
You need 2.
You can buy a combination 1 7/8" open ended wrench on ebay, and cut it in half, but i just sprung for the packing wrenches.