At 170 lbs I wouldn't go any shorter then a 155...
If I were you I would get the 158 if your wanting freestyle riding. 161 if your more on the powder end and 155 or mayyyyybe 151 for pure park
Any shorter then a 158 your going to lose a lot of POW float with your weight, even with the binding set back.
"When I do good, I feel good; when I do bad, I feel bad. Thatís my religion." - Abraham Lincoln