“The meaning of Dia:Beacon — a highly touted nonprofit showcase for contemporary art that opens Sunday — lies in the minds of its beholders. But everyone agrees on one thing: It’s big. Big in terms of dimensions, [and] big in terms of image and ambition.” The gallery, located along the Hudson River in upstate New York, features a jaw-dropping 240,000 square feet of exhibition space, and will feature the collection of the New York City-based Dia Foundation.