I think laying the gun down on its side on a chart would give a general idea as to its dimensions, but considering the breech is wider than the muzzles & the stock is a different width still it would only be approximately correct. AmarilloMike's method would seem to me to be much more reliable. I have just always used the lay it on a table top method myself, though not always handy at a show.

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