OK, on a reasonably maxed out day, where your probably down to your 4.5 / 4.7m sail, how much smaller than your body weight would you go in board size?
If, like me, your were around 97kg would you go for an 83L or an 87L??
I am currently 110kg and usually sail around 84 liter boards with my 4.7 and smaller sails... I probably wouldn't go much smaller in board volume though. I tried a 69 liter board with a 4.0 a while back but it simply took too much effort in the lulls to keep going.
that said I used to sail in the South of France quite a bit and we met this guy there once who was a bit over 120 kg... he had a 69 liter custom paired with a 3 meter sail... at that stage I was somewhere around 94 kgs and was seriously stuggling to control my 3.5 sail and an 80 l board. wind on one day was a solid 50 knts with the odd 70 knt gusts coming through. The French guy was doing mind bending forward and back loops without much problem.
As to your questions... this also depends on where you sail. For example up on Gnaraloo I find the inside is often too light for me to use a small board. There simply isn't enough clearance over the reef to allow me to dog out... if you can I would suggest go to your local shop and rent a couple of different boards in the 83 to 87 liter range... personally I love the new JP 84 twin fins... they seem more floaty than your regular 84 liter board, turn easily and plane surprisingly early. If you want more speed then try the 84 Freestyle Wave. If JP isn't your bag then try the Fanatics Freewave or perhaps the Evos/ Kodess from Starboard.
Sizing up a twinny Grumps? 
I reckon when you're well powered up (doesn't have to be a 4.5m day, can be well powered up on a 5.3/5.5) you can go 10L under your body weight. So if you're 97kg, an 87L board shouldn't be a problem.
Depends where you sail as well...the inside section at Metts is always much less windy than outside the break, and from memory (haven't sailed there for a few years) Scarbs has a bit of a wind shadow as well... it's no fun dogging into a crunchy shorebreak.
The Woodies skate park has pretty clean wind, no drama's there.
My Manta 59 (86L) is 20L under my body weight, but slalom boards tend to have a bit more volume in the tail. Still I can sail it comfortably in 18-22K with a 6.6 (preferably at Woodies...the river is too patchy).
And buying smaller kit is great incentive to drop a few kilos! Not saying you need to of course.
I am 96-98kg and use a 100L FSW with 6.0 down to 5.3. When 5.3 is well powered I'll use my 80L EVO if there are good waves.
On 5.0 well powered and below I'll always use the 80L. However EVO's don't handle chop well and slap a lot, so really I'd be more comfortable on a 85L trad shape.
So small liny little fin and a lot of Vee affect it a lot.
I had a 75L Acid about 2004 vintage and that was only comfy on borderline overpowered 4.5 and below.
The older you get the fatter you get,,where did the fighting weight go??
Working on it right
anyway working on your dexterity should help you sort out the right size board grump
I think you are mistaken
i weigh 87.4kg (as of this morning) and I use a 79l Jp wave in all conditions with all of my sails (5.0 - 6.9) and manage to do ok. I tried using my small board (GEM? custom with unspecified volume but must be in the 60's) the other day when the wind was nuking and found it particularly skatey(I think thats more to do with the board than its size though). I've decided I'll never use it again cause the JP was soooo much better even in the high wind. Interestingly, the small board was my main board when I used to live in Greenhead and I didn't have a problem with it then.....the JP has just made me soft I think.
At the end of the day, its what you get used to.
Weight 90kg
Smallest board 08 JP FSW 92L
Smallest sail North Voodoo 4.5m (very rarely use it)
If I gotta go smaller than that the the go faster kit is a calling
88 to 90kgs 74 litre with a 5.0m down to a 4.0m also depends how choppy it is, big rough chop = small board, not so choppy i can get away with more volume.. but the 74 Jumps like a Demon [}:)]
If you are powered up does it really matter how small you go?
At a svelte 96 kilos, I use an 82L board with a 4.7 or 5.8 but it sucks in anything less than powered up conditions.
In marginal conditions an 82L board under a 96 kilo man becomes little more than a sea anchor.
