The way I see it, the point about foiling is the efficiency and lack of drag minimizes the brute power you need to apply, the possible exception being during water starts (and of course if you want to go really fast or jump big).
20m lines are the equivalent of 23m lines on a LEI, which to me is a normal/long length. If I can't water start with 20m lines I'm nor really interested in creating more of a low end spike by adding longer lines because at that point the riding experience starts to become too marginal.
17m lines are the equivalent of 20m lines on a LEI, which to me is a normal/short length. You lose something at the low end and gain a more responsive, connected feel to the kite. With the 4m and 3m I choose this length. If the wind is in the higher end of the 5m range, I will use 17m lines also. If you're turning on a wave you don't want, or need, the extra boost from the kite -the idea is to use the power of the wave. The few times I've used 14m lines it has noticeably reduced the efficiency of the kite. I can see it (and shorter) for "real wave" foiling though - closer, I guess, to winging.
Having said all that, I was psyched to use the 3m yesterday with forecasts of wind in the mid-20's. As it turned out, it was a rare occasion when the forecast was way under what happened. When I rigged up the wind was around 27 knots gusting into the low 30's. So I went with 14m lines. This was not much of a help as the wind was gusty & the water was very rough inside the "reef" and jacking up to double overhead swells on the outside. It was quite manageable, but not really what I look for in a Peak foiling experience. I came in after an hour as the wind increased hoping that if I sat it out for a while the wind might drop a bit. Instead, it continued to build. By late in the afternoon it was 35 gusting to 42 knots. The guys on SB's and 5m & 6m kites had already packed it in. Couple of guys windsurfing with 3.7m sails.
So ... my ideal Peak wind ranges (around 82 kg):
6m 10 - 14 knots
5m 13 - 17 knots
4m 15 - 19 knots
3m 18 - 25 knots
More than 25 knots - time for the SB.