Thanks to all who replied, I'm now considerably less confused though a bit concerned about impact on my bank balance !
I've a limited range of kit, and I was trying maximise its use - on the day in question, windspeed was around 20-25mph, which from the sail calculator (thanks Screamer) seems about right to be using a 6.5. Just that it seems it was on the wrong board (but it was still fun ...)!
I do generally use a 7.5 or 8.5, and I've no doubt that others were using lower volume boards; I just don't have one yet.
So armed with your feedback I think I have some choices:
1) get a smaller board; (that will have to wait unfortunately)
2) experiment with, say, a 42 cm fin (less expensive therefore more likely)
3) don't sail when the wind is too strong for a 7.5 (no chance!)
Given the comments about recommended sail sizes (6.5 to 10.0 for the 2009 Futura 144, I can't find the figures for the 2008 model), what would be the likely largest sail size I could sensibly use with my board? Apart from the obvious physical challenge to handling a big sail, wouldn't it risk damage to the board going above the recommended range?