The reason that people would be telling you to go with a longer length of rockered ski is because there will be a shorter running surface (length of the ski that is touching the snow) because of the lift of the rocker. Therefore you will have the effect of a shorter ski in some aspects, such as stability on hardpack. Also, I don't believe that rocker adds length to the ski.
Now for what legth you should get, from personal expirience, I got 181s of the Moment reno rockers (I'm about the same height and weight as you). I'm lovin mine, so somewhere around there or maybe even longer, considering what you're already riding.