A larger board will give you more stability, sacrificing a little in quickness. I am 5'8" , 165 # mid 40s and ride a Rad Air Tanker 172. It is still quick edge to edge and provides nice float in the powder. I was concerned that the board would be hard to manage in the bumps and trees but it has been no problem at all. I have not used my 160 Volkl Cross in the 3 years since I got the 172. It was supposed to be my soft snow board but has become my "one board to rule them all".
I tried a wider 177 version of the same board and did not like it. I found the extra width made me work too hard to get the board on edge and was slower edged to edge. Therusty is right that you need to make sure that the board is not too stiff for you. You only really need a wide board if you have big feet. If you are planning on riding in the park and doing jumps and spins, I would stay away from a longer board but otherwise, I wouldn't hesitate to get a longer board. I doubt you would notice the change in length from 158 to 160. That's 1 cm either end. As long as you are within the design wieght for the board, you will be fine. Demo if you can but there is no reason to be frightened of going a bit longer.