I'm 6' and 165lbs (prob closer to 180 with a wet 5/4 wetsuit and booties). I'm riding a 2010 5'10" F-One surfboard (thruster). It's not terribly floaty and I wouldn't ride anything longer. I find bigger/floatier boards too buoyant, making it hard to really lean it over and get the rail sunk for a serious turn.
The "float" of a board has more to do with width, thickness, and essentially volume. The length obviously has effect, firewire makes identical length boards in different volumes to accomodate a riders perfect fit. Like we have seen in the evolution of the twintip moving from long and narrow to short and wide... searching for proper volume.
If you come down a big face a longer board is ideal, in small surf a quad fin short board is just straight up more fun, imo... this is why you need multiple surf boards (i just need more ).