Does this help? Hope I'm not repeating something you've already found with google...
Mod-A-Can Inc. Product Catalog, Arinc 408A Indicator Cases
It appears that "5 inch" refers to the nominal dimension of the can, while the bezel is considerably larger. Your question is how it is defined though, so I fear a mere table of actual dimensions may not help. Bump for an interesting question in any case.