Hi Mantas, indeed, we did used to recommend min and max, but experience and feedback on the forum etc, showed us that customers are often using the sails in smaller or bigger sizes than we reccomended, so we decided to just do a rough max size which is comfortable. I generally find that on bigger free ride or slalom boards (110L +), you can have a "best" range of say approx 1.5-2sqm per board. When you get into the smaller boards, this range will be obviously slightly less, due to the % size (ie, hard to expect a 80, 90 or 100L board to work great with 4, 5 and 6sqm sails equally well - will always be a bit more slanted towards certain sizes.
Nevertheless, I would say on the FW STB 95L for your weight - ideal sail size is probably 5.3/5.7 for winds of 14-20knots. After that, you can use smaller sails too, I have used the FW STB in Tarifa (same weight as you approx) in winds over 50knots..:-) it "works"....generally you'll find with all boards big or small - when the sail is too small, the classic feeling will be that you have a quite nervous sail feel, often over-sheeting the boom in, an irregular acceleration and often more control problems than with slightly bigger sails.
Some may disagree, but I feel like under 100L size, you don't really want to expect "perfect" performance in more than 1.5sqm range per board. For example, on a Grip 82, my favourite shape, I accept a slightly worse feel with a 5.3 if the wind is very light, perfect on a 4.0-5.0, then on a 3.4/3.7 the board really hits it's limits in terms of size, but is still fully possible to use?
Not sure if that helps?