It is the smallest square socket, on one of the short sides of the Tee. Here it is alone on the valve stem:
And here it is connected, passing through the lock nut wrench:
The square socket on the long end of the handle is much bigger (see it in both pics). I hope that fits the Honda Civic (plenty of room/clearance).
I didn't bother to remove the valve cover or the hoses on the "EGR valve", but if I do, there will be ample room for adjustment. At eight bucks, I might ordering a second one and just cut off the other side of the Tee handle for easier access under there on one of them. The spare Tee will be for in case I need the slightly larger square socket on the other side of the Tee for something! Sweet inexpensive tool.