Odd that a 3 year old thread suddenly pops up ?
Anyways, to answer the question, no, longer lines do NOT mean more power as such, so the "3 meters equals one kitesize" or similar is simply bull....
Longer lines means you get a longer reaction time, so for riding waves it is not desired.
For hydrofoiling it means everything, as you get the longer powerstroke thus the ability to get up easy, without getting overpowered once up where you dont have any drag nomore
Also great in transitions where you get power a lot longer thus the ability to foil or plane all the way through the turn
For normal powered freestyle and freeride, it is usually not good with longer lines, too long reaction time before it changes position in the window
Of course, if really underpowered, longer lines will make you able to work the kite much better - but this is hardly the desired scenario
Peter