I just picked up (yesterday) a brand new pair of iMonster 78s in 177. I weigh approximately what you do and ski aggressively (for someone over 40). I demoed the ski in both 171 and 177 at Alta's demo days last April. I would not have skied the 171 at all but it was the only one available when I first showed up at the Head tent. Obviously, I bought the 177 because I think it is probably a better fit but was surprised how "long" the 171 skied. I did not ski the 183 at all but was not longing for more ski when I was on the 177 so I went with it thinking that I would trade marginally more manueverability for marginally less stability. It is the first time I have bought a ski in less than the longest length available for more than 20 years.