27mm
theoretically!
but practically, as I said before, they often swell up, so I use the next size up imperial socket. 1" 1/8.
I know that is the equivalent to 28.5 mm roughly but unbelievably I even have one nut on the car that is too big for it!
Have you gone around the car to see if any are too swollen even for that? As I advised you in another post?
And did you buy the one we all advised you about?