How large and heavy is the B737's built-in stair?
A number of 737s are fitted with forward airstairs (and aft ones too). The Boeing Airplane Planning Document gives the airstairs opening as 0.41 X 1.12m, while the airstairs can be folded to less than 2.5m length. This should give the volume occupied by the airstairs.



Image from Boeing 737 Airplane Characteristics for Airport Planning document.

According to an article in aviationpros, the total system weighs in around 200 lbs:


  Aircraft such as the DC9, MD80, MD90, and 737 aircraft have had a long enough history that many of them were built with the forward passenger air stair assembly. Many aircraft were also converted to non-airstair versions Ñ a 200+ lb. weight savings for the operator.