Standard sizes and dimensions for Bilco's Classic Series basement doors include widths of 47, 51 and 55 inches and lengths of 43.25, 58, 64 and 72 inches. Heights range from 19.5 to 52 inches, according to the company's promotional literature on door sizes.
Homeowners determine the proper door size for a sloped basement door by measuring the areaway foundation. Individuals measure the inside and outside widths of the foundation, as well the length from the wall to the edge of the top step and from the wall to the ground. The frame of a Bilco door overlaps the foundation by 2 inches on each side, and the company makes foundation plates that reduce the inside width to ensure a proper fit.