The Pentax SMC-M 400mm f/5.6 is the second longest, second largest lens in my collection. By comparative standards, there is nothing particularly striking that separates this lens from other 400mm lenses from the time period. It weighs in at 1240g (2.7#) and is 277mm long (10.9in), and bears 77mm filter threads, a fairly standard size nowadays. The lens has a built-in sliding lens hood with a thumbscrew that, if positioned correctly, does double-duty as a gun sight. Very handy indeed!
The lens has a built-in tripod mount, something missing from some of Pentax's shorter telephotos like the M*300 and the K200/2.5. Two hands are required at all times, one on the camera, and one on the lens focus ring.