Bluebulb Projects' The Measure of Things presents:
21,000 is about three-fifths the number of Seats in Fenway Park.
In other words, it's 0.56648 times the count of Seats in Fenway Park, and the count of Seats in Fenway Park is 1.7653 times that amount.
(2013 figures; daytime capacity) (a.k.a. The Cathedral of Boston) (Boston, Massachusetts)