Subway posters are huge, limited edition posters printed by the record company to promote a new album or single at the time of release. They are typically seen on billboards, subways, and music store windows, etc. 
All the posters in this gallery measure 60 x 40 inches, except the "We2R1 LP" poster which measure 40 x 28 inches.
The picture below shows the scale of the poster against a 12" vinyl.