Like always, my personal and humble opinion.
So you're not looking for any tricks, you are talking flat water, and you want an early planer to blast around in light winds, forget the carve and go for an Isonic.
With your weight of 73kg, I would not go bigger than the IS117 (75cm), perhaps the IS117 wide (80 cm) if you strongly want to point upwind.
Don't pay too much attention to the volume, it's the width that counts. This sounds like a cliché, but it's the truth, especially for a light weight guy like you.
The IS117 will plane much earlier than your 120L board of only 68 cm, also because of the wide power tail of the Isonic. For your 8.5 sail, I would suggest a fin of 46 or 48cm.
I'm using a predecessor of the IS117, i.e. the IS122, with an effective volume of only 114L and a width of 75cm in light wind conditions with sails 8.4 & 9.5. The 8.4 sail is right in the sweet spot of the board and I weigh 86kg.
Bigger boards will only make life more difficult when the wind suddenly and severely picks up.
Also to be efficient, bigger boards need those bigger sails where you are afraid of. By the way, I can understand this, when the wind force suddenly triples.
Those bigger boards only help going extremely upwind or in ultra light winds.
Hope this helps.
IS87 / IS101 / IS122 / US147
Last edited by BelSkorpio; 16th November 2010 at 02:15 AM.