What part of the world are you riding in? Also "all-mountain" is a bit of a generic term. If all mountain for you is groomers, some ice, moguls, the occasional few inches of fresh snow, and some park, a shorter board would be okay. If all mountain can include some steep off-piste runs a shorter board may get overwhelmed for your weight. You're not a small guy.
FWIW I weigh 175 and just bought a 160 cm board with a 140+ cm effective edge. So I guess it "rides" like a 170+ cm board if you think about it only in terms of effective edge.
It certainly took some getting used to riding something with 8" more metal touching snow on either side, but I have now taken it through nearly everything I took my old 159 cm, 123 cm effective edge board through, and it's not some scarey beast to ride. It's so confident at carving that it's making the rest of my boarding even better, so embrace the length, embrace the effective edge, and don't sell yourself short!
HA! I made a funny... lol