see this is where im a noob! Im big broad shouldered an tall! wasint sure if a 155 would be too small or not!
too small for what though? its not like the board is going to blow up if you get one too small, you can ride any size you want. sure if you're a big guy on a little board there is going to be some compromise with stabillity at speed ( esp. if its soft as well) or in deep snow. if you are a little guy on a board thats a bit bigger than what you normally use then there will be a compromise in the other direction, it could be more stable at high speed and harder to freestyle.
if you took 10 dudes that where 5'8 and 160 pounds (just as an example) you'd probably find they ride different sizes of boards and do fine on them, there is no rule to what size you need, and 90% of all boards sold are between 152 and 162. thats a 10 cm range, not a big deal.