are you getting new-gen boards or trad/classic?
JP's I'm guessing?
If you're going the RWW then ~82L should be spot on - 58cm max width. My evo 80L goes great with 5.7m sail for the lighter days (I weigh 78-80L wet).
But the classic boards tend to be narrower, so if you're looking at Pro Wave your effective planing area is reduced and a few extra litres will come in handy. But the JP Pro Wave only goes up to 79L

It looks like they have made the FSW more wave orientated this year, so maybe the 84's the go if you favour jumping over riding.
Of course, I don't think you should be buying any JP board, but whatever blows your hair back...