Hi Chris, for your weight, the 1000 would not be an issue, especially if you often have 15-20knots. The gap is fairly large between 1000 and 1750, but again, if you are gybing well and prepared to pump a touch more, then the 1000 will be a better fit for the windier days. Other option is the Carve 2.0 1100 which fits right between the 1000/1250 - has great low end and still very easy to ride, will be faster and better overpowered than the Free 1250. Glide 1085 is also a good option, but I think you will be overpowered quite quickly at 18-20knots with that.
Long term at your weight and with a small board, I would imagine you would be on a 1500/1250 as the biggest front wing, with a 800-1000 as the smallest. I'm 76kg and ride mainly 850/1250, also mostly fresh water/lakes?