Thirty-eight
The standard US military sidearm for many years was a .38 calibre revolver. In the early 1900s the Colt model of 1905 and later the 1911 replaced it because it would not stop Muslim fanatics called, I think, Juramentado. These used to enter US military areas in the Phillipines and do serious damage before noticing multiple .38 calibre hits. In this case size does matter.
While I see the rationale behind many of the anti gun in the cockpit arguments, I cannot think that there are many pilots who would not wish to be armed when the b******s are at the door.
Obviously preventing them from getting there should take first priority.