I use a 31 cm G10 Volt on my 65 cm wide Manta with a 6.6m and a custom extra wide 32cm G10 Volt on my 70.5 cm wide 116 litre Manta with a 7m.
Both go upwind good to very good with little spin out problems.
I previously used a carbon 35cm Sonntag SLR with my 116 or a 38 carbon Volt.
The difference is when you're powered up the smaller fin has a nicer more controllable feel and it's faster too.
Downside - when you're sailing underpowered it's harder to get upwind.
The other day I changed my 32cm Volt to a 44cm carbon Volt,also dropped the downhaul a bit on my 7m and was able to keep sailing powered up for another hour. Wind was averaging 14 knots.
It confirmed to me bigger fins are amazing upwind and for planing thru lulls etc, but they become a hindrance to performance if there's enough wind.
I sail my 137 with a Volt Carbon 48, top end is down but it points so high I just love it. Just to be cheeky I'm chasing a 44 Volt Carbon if you're thinking of selling ??
Select to expand quotepetermac33 said..
It confirmed to me bigger fins are amazing upwind and for planing thru lulls etc, but they become a hindrance to performance if there's enough wind.
True, but there are few things nastier than having too much fin when the breeze kicks up 
Look, small fin on a big board- people may make fun of you , but really its how you use it.
Just ask the twins I have been with.
Its surprising how small a fin you can get away with. In shallow weedy water I use a 25cm weedy on a 114 slalom board with a 7.8.
On a 99 ltr board I get away with a 19cm with a 7.0. No problems in chop (unless its scary big), upwind or deep bearaways
But in normal water conditions pointers are the go. Its how you use them with your gear.
I had a session on 66 wide with 6.7 sal & 21delta. Up wind was just ok but off the wind was great. Too weedy for a pointer
my usual with that kit would be 25-33 WS
Maybe sailing multi-fin wave boards can make you faster when you get on your race-gear, by forcing you to be light on your fin / fins. You can't sail a wave board with very much back-foot pressure as the fins will give way, so if you apply this technique on a slalom board you may be able to get away with a slightly smaller fin which is both faster and easier to control.
If you're pushing hard against your fin you might be in control but you're not going fast.
^^ No doubt there are better people than me to advise you, but one of the keys to speed is to make the board travel as straight as possible, which reduces drag from the fin. To do this you don't want to be pushing on the fin, you just want to provide enough resistance when it pushes you via constant tiny adjustment.
Getting this right takes a lot of practice of course, but maybe it is something to work on and using a smaller fin is a way of making you do it.
This is Finnish windsurfer Tuuli Petaja. She's quite small and is on a big board. Seems to work fine.
Select to expand quoteAUS1111 said..
If you're pushing hard against your fin you might be in control but you're not going fast.
I use 24 inch lines and sail with a very upright stance putting as much of my 70 kg as possible thru my back foot on to the fin - ONLY WAY TO SAIL or so I thought!
Finally figured out why Isaac flew past me yesterday.....
I had a try of his gear and his 30 inch lines were way,way too long and I had trouble powering up my harness. It felt very physical to sail despite his sail and board being the same size.
He also overtook me on my gear but he definitely was not sailing it as good as his own.
Too late to change,but maybe the change to smaller fins will help.
If you are going to try a much smaller fin than is recommended for the board - firstly make sure there's enough wind to be powered up. Also try to use a small fin that is wide and stiff. It will reduce the feeling of going sideways.
I don't understand if you can get away with a smaller fin and are well powered up why not use a smaller board as well?
Good point powersloshin,,I've been thinking the same thing myself!
Still nice to experiment.
Using longer lines and getting your weight off the board might be faster but....
in really strong conditions,keeping your weight over the board with an upright stance should result in more control therefore more speed?
Select to expand quotepowersloshin said..
I don't understand if you can get away with a smaller fin and are well powered up why not use a smaller board as well?
I guess it depends what you are up to, but in slalom racing small boards accelerate too slowly out of the gybes and you generally won't get that back on the straights.
In a longer race like the LOC, acceleration is not really a factor so you want whatever you can control the best, assuming the wind is steady. Having too big of a board is always gonna be less of a problem than having too little - at least you won't be underwater!
In light wind and shallow water with weed I've used a 24 delta on my formula and had a good time I just moved my straps all the way inboard !
This is interesting, for me (70kg) on my 110 falcon in small harbor chop on a 7.8 or 6.6 I find my 42cm venom is 2 knots faster off the wind and up wind than my 38 or 36 Vector volts. I tried a few sessions and kept getting the same result so the 42cm is my default fin for that board. A weird thing is the 42 is slower in flat water than it is in chop by about 1-2 knots?
I'm yet to try the smaller fins on this board in flat water as its an upwind mission (eg 20 tacks using the 42cm) to get to the only flat water available.
I feel pushing on the fin helps the board to fly and thus improve speed. The comments here have encouraged me to give the smaller fins another chance and try less back foot pressure to see if that helps. Good to have some new goals the next time I'm out 
I find using a smaller fin u have to use the rail of the board more , lengthening the water line.
Pushing hard on a bigger fin seems to shorten the water line and gives it that lifting faster feeling. Canes the back leg though !
