I don't do rails, but about everything else. been riding a 158, can get a 158 or a 161 (i'm a dainty 270) and I'm gun shy on what size to order up.
270??? I'm 250, and my smallest board is a 171. I would never even consider anything under 165 if I were to ride park/pipe, which I don't. But hey, it all boils down to what works for you. I been itching to demo a Rad Air Tanker 200. Yes, 200 cm!
I'm 240-250 and my regular board is a 165, park board 159 and I was thinking of getting a 156W for a park board as well. Whatever you feel comfortable on. If you're not nose pressing out of a jump or really laying heavy on it you should be fine. If your looking for more spins and forgiving landings (as in easier to maneuver at that last second), the shorter you go the more you get.
I am 165 and I ride a 159. It's a bit longer than I'd like, but not by much. With the exception of rails, I ride everything. I like the longer, stiffer board for jumps. I can't imagine you should be riding anything shorter than 170s at your weight if for nothing other than fear of breaking it.