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.