Honestly I think the advice that you are getting is spot on. My friend is a Pro rider and he said that the rider weight is really what you want to look at.. also boot size.
For Instance, I am 6'4 195lbs size 12 US boot and I ride a 162cm Wide board. Now I could go down to a 161cm or up to a 163cm but no more then that.
I think the real question you need to ask yourself is this.. are you going to gain a significant amount of weight during the lifespan that you plan on riding this board? If so I would size up a bit.. to put this in perspective your thumb nail is about 2cm, thats the menial size diffrence we are talking about here.