The Formula for Sail size versus sailor weight versus board volume :


Sz x 2x - Weight divided the the volume of the board, multiplied by the square of the wind velocity which is divided by the volume of the board, the halved over the square root of the sail size which is xmeter squared, this is then rotated through an orthogonal rotation to determine the leverage factor, given the height of the sailor, and the boom, height and it's leverage through the mast foot, now this is also disturbed by the sea state so choppier water will require a factor analysis, for every 100mm of chop you need to determine the swing weight of the board which affects leverage through the board, and speed which you divide by the number of forward pitches per 100m distance travelled, a bit complicated but I hope this helps.

Jimmy,

I would say drop 1m for every 20kg less of sailor.

So on the same board, you would be looking at around 5.8m to 6.0m sail for the 65kg sailor for the same wind conditions.

Also bear in mind sails can be rigged differently to offer more or less power. (you may already know this). So you don't have to be that exact with the size.

Usually, more downhaul and more outhaul will flatten the sail and make it less powerful.
If the sail has got cams (slalom sail) taking them out will reduce power. These adjustments can make big differences.

using hardies formula i worked it out to be a 6.5 for the lighter sailor

Well I bet you're confused now Jim!!
(There is no spoon)

I assume you are a 'weekend'/'general' sailor.
After wading through all the wishwash, id still get a 6.0m for your lighter friend.
Only one other person stuck their neck out and said 6.5m. So why not compromise and go for a 6.3?

This assumes you both have.
Same board
Same mast
Same boom
Same conditions ( i.e wind)
Same proficiency of sailor
Same Star Sign
Same Hair Gell

and....
the 65 kg person wants to do the SAME things as you do on your 7.0m
I.e Assuming all things EQUAL.

When the wind picks up significantly I usually put up a smaller sail. Same would apply if I realised I was carrying 20kg of lead in my wetsuit and then decided to take it out.

After saying all this I notice you havent posted a response so I might just be talking to myself!!

At the risk of over simplifiying things, If all other conditions are the same, could I suggest that sailor weight & sail size have a linear relationship.
ie, a 10% increase in sailor weight should need a 10% increase in sail size, (and perhaps a 10% increase in planning area????)