The length of your harness lines depends almost entirely on how tall you are.
How much of you is legs.... how much is torso.
You need to be able to make the "figure 7" stance, i.e arms pretty much straight out,
hips well inside the shoulders.
Think of yourself as a cantilever beam from footstraps to shoulders.
Your harness line length needs to be the length that will hold your butt at the correct level and angle to get your upper body weight as far off your board (to counter balance the pull of the rig) as possible.
How far apart..... lots of different opinions here. I use the width of one fist between the attachments. Some of the better young freestylers put them totally together.
What you don't want is to get them so far apart that they pull you forward and back.
The idea that you can get harness lines that are too long and simply spread them out does not work for me, but I see sailors doing it all the time. I'd get ripped off the board in the first big gust.
Getting them correctly balanced is absolutely critical.
You have choices here.
Do you want them to sheet out slightly if you release your back hand..... stay neutral if you release your back hand, or (heaven forbid) have them sheet in some when you release the back hand.
The latter is actually done by the formula racers (especially the smaller/lighter guys and juniors) as they set the rear line as the "pivot point", and do all the sheeting in and out with the front hand. But they are on huge sails and max overpowered alot of the time.
So, there's no "best' way.... you have to work out what works best and is most comfortable for you individually.
I might take your rig for a spin and come in and tell you your lines are too far forward/back, but they balance nicely for you.
You could get on my rig, with sails that I can let go of with both hands (rig balances on the harness lines and does not sheet in or out and does not fall forward or back and I can sail along without touching the boom for maybe 30 seconds) and feel the rig is very much out of balance.
I find the "sailing without hands" using a seat harness, with sails larger than 5.5 m2 works pretty well.
When the wind gets up, and I'm doing B&J type sailing with lots of chop and ramps, I prefer a waist harness as I can move around more, and the stance on the smaller board and fin is much more erect and over the back of the board.
Hope this helps,