RE: Formula Sail Sizes?
I guess a lot depends on how variable and what your general conditions are like. If you pretty much sail in the same winds all the time you can easily be successful with one sail; but if you're experiencing conditions between 4 and 12 m/s and everything inbetween then 2 and maybe 3 sails could be necessary.
I originally wanted to get by with two sails (both logistics and cost seemed nice) but ended up with three - mostly based on what was availible at good prices. I have Vapors 12, 11 and 9.5 - originally wanted 11.5 and 10. The 1 have massive power, something that does come in handy as I often sail in just around 5 m/s which means that the 11 will plane easily, but have hard time getting me upwind. The 12 on the other hand is very very nice in light winds. I pretty much never use the small sail; but figured that quite often when the wind is too heavy to hold the 11 water state means that control is easily as important as power, so when the 9.5 came up for sale (used/cheap) I grabbed that. I'm 95kg and was on a 159, now a 160.
That's a bunch of ramblings that might not be helpful at all?
Are you planning on buying brand new sails? I don't know where you sail, but around here it is my guess that anything but maybe the national top three will be within reach with 2006 sails, even next year. And if you haven't got a good deal new sails are very expensive, whereas 2006 sails are getting pretty cheap (relative..) right now.