As the old saying goes there are three ways for everything, The Right Way, The Wrong Way & My Way. Well My Way is if someone asked me what size shotgun I had, I would give the "Gauge" number ie 12 gauge etc. Should they then ask, how do you know that's it's size I don't see a mark on it to say (Most old Classic American doubles are unmarked), I would then say that's easy I measured it with a "Bore Gage".

Miller/TN
I Didn't Say Everything I Said, Yogi Berra