188 is a hell of an off piste length for you, it'd be perfect as a big mountain style ski. I'm the same size as you and ride a 178 and a 185. That's tough because you should look in the 180 to 183 range.....right smack dab in the middle.
I ride a 178 most of the time (volkl G4), ski super hard and that seems to be enough. With your weight, I would think you'd be okay with that 176, you're really not that heavy and the ski is 88 underfoot, plenty of float. And the shorter that ski is the easier the 88mm will be to turn over on piste.
SO, my .02 is you go with the 176...