Attached Link: http://www.msichicago.org/scrapbook/scrapbook_exhibits/bigtop/big_timeline.html

"Why is the central performance area "ring" shaped? The central performance area was made into a ring shape for several reasons. Its shape permitted audience members to see the performer at all times. Through centrifugal force, the performers were able to maintain better balance while standing on the back of a horse. After some trial and error testing, Astley decided on a ring diameter of 42 feet, which has become the international standard for circus ring size." I guess 42 feet is the standard. I checked 3 sites and they all had 42 feet, so must be a decent size.