Bluebulb Projects' The Measure of Things presents:
15,000 is about two-fifths the number of Seats in Fenway Park
In other words, it's 0.40463 times the count of Seats in Fenway Park, and the count of Seats in Fenway Park is 2.4714 times that amount.
(2013 figures; daytime capacity) (a.k.a. The Cathedral of Boston) (Boston, Massachusetts)