Depends on what you want to do to your ride. OEM is 12.5" I believe. Going up in length increases ride height (and ground clearance), shortens trail and improves low speed handling. Going down in length increases trail, improves straight line handling and lowers ride height (and ground clearance).
If you're short legged going down 1/2" is OK to lower the ride height without too much impact on steering or ground clearance.
Hagon's can be bought at 13.4" and is the MAX length you can put on.