I'm 5'8" and I ride either a fat 153 or a skinny 156. Ideally I'd want something around 158 or so... being 5'11" I'd reccomend at least a 160, probably more like 166 or so.

I find the longer board makes blaring through choppy surfaces much easier and more stable. Most modern 170s are lighter than a 145 was three years ago, so I wouldn't worry much about rotation in the air. Turn in and tight manouvering wont be as precise, but if you spend most of your time on groomed/semi-groomed runs, you'll love the extra length.

My personal solution is to just have several boards (I own 4) and choose the one most approprate for the conditions and the type of riding I plan to do.