The newer subways are usually between 45"x58" and 46"x60", in the portrait format. As FA said, many are basically a big sticker. The print quality is not as good as bus shelters and one sheets, and personally, I don't like the sticker type vinyl material.
The older, half subways are cool, IMO. They're usually about 29-30"x45", in the vertical format, from the same or similar stock to one sheets of their time, and much rarer than one sheets. They are like one sheets on steroids, and there are many from the time just after the NSS closed shop. In a sense, they are similar to the old 40x60s, and current bus shelters, as they feature the same artwork, just in a larger size than one sheets.