I would caution against using your shock to limit down travel. Over-extending a shock can damage it just the same as over-compression. In my experience, the control arm and track bar geometries limit the down travel before my springs want to unseat, so I got the longest shocks I could fit with the amount of bump stop I was using.
The appropriate length is determined by the compressed and extended length of the shock compared to what the rest of the suspension is capable of. If the compressed length is shorter than the bump stops allow and the extended length is longer than what the coils, control arms, and track bar allow, then your shock is safe.
Ok.
Edit: I couldn't leave his response unanswered. I tried...haha.
Sounds intelligent enough but is hardly true. Down travel will and should be limited by one of two things, shocks or limiting straps. Using the track bar and control arms will cause far more droop than other components are capable of ie...drive lines, clevite bushings, causing binding which could lead to failure.
While I agree if I were bombing through the desert with my vehicle constantly unweighting and/or leaving the ground causing my wheels to extend to max down travel, "topping" out the shock sharply at high speeds (shock rebound speed not vehicle speed) can cause damage. This is why trophy trucks use high and low speed rebound damping and limiting straps.
My 2012 JKUR is hardly a trophy truck so I don't feel me utilizing my full down travel at crawl will cause significant damage to my King 2.5's when they fully extend and limit my axle droop. Again with shocks too long and sway bars disconnected I'd be more concerned with my coils unseating and/or falling out while flexed and full down travel is used.
I'm sure someone with much more experience could chime in and correct me if I'm wrong.