@14knot w/ 72kg and arx 15.5 anything works
length is not important, neither volume.
you don't need sharp rails, only thin ones!
the width and rockerline decide wether a board starts gliding early or not.
if you have 12L volume in your 1.60 board or if you got 4L in your wakestyle - both wil instantly sink when kitepower's missing ...
boards like the x-shooter (or similar concepts)
work great for this - the flex will increase the rockerline while power increases - so you have an early gliding board with lots of rocker, when needet.
in 14+knots, small wakeboards work as well.
depends on your opinion - wether you need an aditional board (for your directional)
or if you want only one that fits all conditions.
i persanally am riding a 1.34 wakeboard and never want to have anything bigger
it's so comfortable - it fits everywhere (in my car), no problem to carry arount even in strong winds ...