I’d buy about 6.5 - 7 feet, or 2 meters of the rope from your marine supply shop of choice. Here’s the instructions from there:
1) Depending on what year / model of SLE bar you have you will either be tying the one end of the trim line off on the (‘07 or ‘08) Chicken Loop or you will need to tie a loop with a stopper ball and knot to terminate the one end of the trim line to allow for a leash attachment point and prevent the line from pulling through the (‘09 or ‘010) Chicken Loop. Before you remove the old trim line take a look at how it is set up to determine what style of trim line you need to make and how it is terminated at the end opposite to the cleat
2) Once properly terminated on the end opposite to the cleat you will need to re-assemble the bar. On ‘07 and ‘08 bars that have the trim line tied to the chicken loop this will include a below the bar spacer to balance the load. On ‘09 and ‘010 bars there are no additional parts below the bar.
3) The end of the trim line you wish to thread through the stopper and swivel assembly must first be cleaned up to allow for it to be threaded. Your kite should include a patch of sticky back Dacron. Cut a 1"x4” strip (3.5cmx10cm) of Dacron and carefully wrap this sticky back Dacron around the last 1” / 3.5cm of the trim line such that the remaining 3” or 7cm of Dacron hangs off the end and can be compressed into a point to allow for easy threading. Minimal overlap of the Dacron on itself is important to keep the overall width of the trim line from growing too large at it’s end
4) Thread the trim line through the various parts of the bar and back to the Chicken Loop. TIP - The stopper ball has larger holes on the TOP than the bottom, so make sure the stopper is orientated correctly
5) Thread the trim line back to the Chicken Loop and down through the opening and cleat.
6) Here’s the answer to your question concerning length You should take enough slack out of the trim line so as to ensure that all 4 leader lines are even. You don’t need to get overly technical at this point, just ensure that if you were to nail all 4 lines down on a board so that they all extended in a straight line towards the kite (as opposed to pulling them all together into a bunch) that they all terminated at roughly the same length.
7) Slide a stopper ball onto the trim line and tie off a knot to terminate the trim line at the cleat so that with the trim line at its longest length (full power in the cleat) all 4 trim lines terminate at the same length.
8) Once this process is finished it is very important that you finish the job by running all of your lines out and checking their length. With the bar at full power and the trim all the way out all 4 lines should be the same length. It is quite likely that your front lines will have stretched so slide them down a knot or the back lines up a knot to even your line lengths out.
In summary If you intend to replace your trim line - be it with one of the ones we supply or with your own home made line - it is important that you set the trim line length such that the leader lines all terminate at roughly the same length. Once this is done simply run your lines out and check to ensure that all 4 lines are the same length, adjusting the relative length as needed in order to ensure your kite is not under sheeted and prone to back stalling.