Davide,
I don't have a boat of that size so can't really say exactly which servo to use. In general, the bigger the better, sort of. Depends a lot on the weight of the thing, and it's price!
There are a number of ways of 'multiplying' torque. 'Guiri's method certainly works. There are disadvantages, as in the amount of travel available, but that can be 'got around' in most cases.
Using a sail winch is a possibility. (Servo makes more than 360 degree turn.) Winchs have disadvantages too, a bit more complicated to rig, and again, cost. [Not sure which is the correct spelling, 'winch' or 'wench'. Same disadvantages apply to both, complicated and cost .]
The attachment point on the sail's boom.
Just depends on how much travel the sail should have, and the amount of line the servo can give to produce that amount of travel. The 'right' attachment point is where those two 'needs' balance each other, sort of, if that makes sense. Then again, you have the option of using other variables to get that amount of travel necessary (where the line is attached to the servo arm). As long as the boat isn't for a particular porpose (racing where design is 'fixed') none of this is carved in stone. If it ain't 'right', change it.
When you get to the point of totally hating the whole thing, beat it to death with a large rock. Very satisfying! Then build another...
- 'Doc