It is really a lot more complicated than that. You could tie yourself in theoretical knots, but why not get practical. Hook a simple spring balance onto the end of the sheet and measure the tension. Note the wind speed. The force on the sheet will vary with the square of the wind speed. EG double the wind speed and the force will increase by 4.
The force is proportional to the area, so double the sail area and you will have double force.

All approximations, of course, but better than trying to do pure theory.