Hey Richard,
Yeah small boards make jumping in flat water a LOT easier...
it just makes landing and then sliding a bit more difficult...
I think with 80 kilos you might end up on a 100 litre board... its just more fun and feels smaller... it is planing a little bit later, but for sure it pops better and also has a wider range when the wind gets stronger...
Maybe go and try a buddies 100 litre freestyle board on your big sail, then you can say more... its hard to tell what suits you better... but a smaller board usually is more fun to turn and easier to pop... with 80 kilos you are not that heavy, so 100 liters could be prefect...
all the best
Rossi