60mm ID x 210mm Length
Why Variable?
Softer ride for more plushness in the beginning of the stroke.
Slows compression on a big hit keeping the bike from going too deep into the stroke. When you go too deep the spring will over power the rebound circuit on rebound and allow the rear to kick up abruptly.
Gives a larger weight range when adjusting your spring preload adjuster for luggage or passenger. This adjustement will keep your bike steering precise and neutral. 
Keeps your shock cooler for longer lasting shock and fluid.