I use
27-28 m on the 8 m2. With 21 m std lines it doesnt really work IMO, not more low end than the 6 and it turns a lot worse. 28 m and you can cut the lowend 1-1½ knots down.
21 m on th 6 m2. Longer doesnt really help with lowend, as it turns fast and accelerate much better than the 8.
21 m on the 5 m2.
21 m on the 4 m2.
19 m and a shorter bar on the 3 m2. Have used 21 m and std bar, also working great.
Tried 16 m lines on the 3 m2 for a handful of sessions, useless IMO as you lose all range in turns and waves, and no advantages with the shorter lines.
In fact, this also goes for LEI's, really small kites works better with standard or longer lines - one can shorten them on the medium sizes if desired.
Quite opposite to what most think (for some reason I dont understand)
Peter