Grantmac said..I think I was using ~30" (adjustable) lines then on a board with the front straps outboard and a standard diameter boom.
Now using 32" fixed, board with straps on centerline and a skinny boom. Plus I now use a hook that adds about 1" of effective length compared to my old roller bar.
I'd say board width might actually be inverse to line length but I haven't foiled on anything really wide in years.
In my experience on larger boards I can more efficiently use shorter lines. I ended up using 30" on both my racing setup (1m wide & 10m) and freeride setup (3 strap 70 wide & 3.0-4.9).
For the racing setup in really light wind I might go shorter (about 28" minimum) to allow a more efficient 7-stance. In high winds I might go a little longer (32" max) depending on the chop/ waves and gusts. The gustier the longer, the wavier the closer to 30" I stay.
For my freeride setup I can use anything above 30" comfortably, dabbled with going to 34" for more freedom, but found that beyond 30" not much changes in terms of freedom, but I feel less efficient in lulls and more often accidentally hook in. Shorter than 28" really feels too short for me, no freedom of movement whatsoever and too direct connection to the sail in gusts. Ended up with 30" fixed lines.
Tbh its the same as wave vs slalom. Wave is close to the centerline of the board, the pro PWA guys are on 32-34" lines in general. Slalom further out, most are on 28-30" lines.
My advice remains the same, go shorter if you feel like it now, as most of us did that while learning. Just dont be afraid to lengthen them again if you're comfortable and are able to relax your arms a little more. A little longer lines (around 28-30" is optimal for most people) allow for a more efficient 7-stance to enable light wind riding and allow for more control in gusty weather. In the end its also for a big part about detaching your body weight position from the rig position unlocking higher speeds and control.