Note the author of this article, David.
Mike Valenti, a long-time member of this forum, is a well respected faculty member of the BMW College of Knowledge and has even been shanghaied by The Snowbum to produce some extensive Airhead tech articles.
Most everyone will tell you that Mike's knowledge is golden. As for me, what Mike says has always been most excellent advice
Show of hands... How many of us have benefited from the Sandy Eggo man's help? I'm a multiple winner myself.
BTW, when you get down to this small size of an o-ring, it's almost impossible to detect any size difference; especially when it's going to be clamped between the caliper pieces.