The "Davy Crockett" was a recoilless rifle roundwith a nuclear warhead. Casing diameter was 11 inches at its widest point; length was 30 inches long. Yield was variable from 0.01 kilotons (about four-five times as big as the Alfred P. Murrah Building truck bomb) to 1 kiloton (400-500 times as large as the APMFB device).
Something like that would fit inside a file cabinet. I don't see how you could check everything closely enough to prevent a cagey opponent from smuggling in a small nuke--one could simply hide it inside an electrical transformer casing with layers of boron-impregnated foam rubber and lead to stop the signatures from leaking out.
I'm going to have to find Erwin Strauss' book, Basement Nukes: the Consequences of Cheap Weapons of Mass Destruction.
Sooner or later we are going to have to quarantine the Muslim Islamikazi world from the civilized world.