I just started work on an ST1 and an SR1 last week and I was faced with the same dilemma.
According to my measurements the nut is 56mm, but my local hardware store doesn't stock that size. I tried a 55mm but it was too small.
On YouTube there is a video where a guy uses a 2-1/4 socket to loosen the bolt on an LD1, so I bought one of those.
It fits a bit looser than I would like, and I haven't actually used it yet, but hopefully it will work for me too.
Video here
https://www.youtube.com/watch?v=2HUhSHeK5kc&t=194sFor some reason the video doesn't seem to start at the beginning but you can back it up using the mouse on the progress bar.