RE: Freeride sail options
For further consideration: Certainly a 6.5 makes sense for 13-18 knots - you should fly @ 18 knots. Modern sails & boards have quite a range. If 6.5 is your starting point, I'd suggest the next size up be no less than 8.0. Problem of course is typically 8.0 needs a 490 mast (always go with the recommended mast).
The bigger the sail, the biggger the size gap you can have (I go from 6.5 to 8.5). The 6.5 on 145 litre will go easy @ 14-15 knots. When you get into ligher winds, the wind power loss is large & the wind holes can be big.