In order to set the length of the shock (Preload Adjustment), you need some help from some friends and a tape measure. The preload is decided by how much static sag you want.30mm - 35mm of static sag is a good starting point. The sag should be set the same front and back to ballance the bike.
Please read this to help you understand how to set the static sag:
Remember when doing this you must be in full riding gear when sitting on the bike in order for the settings to work correctly, as the sag setting is affected by your wieght.
Your wieght being the major factor for setting up the sag, means different preload settings or shock lengths for people of different sizes and wieght.
Also the preload adjustment dose affect the ride hieght as Twisted has pointed out, so keep this in mind when you adjust the remaing ride hieght required with your ride hieght adjuster.
I hope this helps, but if it all sounds to hard get a pro to set your bike up for you.
p.s. Bike setup is also must take riding styles into account.