i have 6 " of stroke and a 12 " spring..Depends on how long of a spring. The longer the free length the lighter the rate you can run. Also need to be aware of the shock length too. You go too long on free length and by the time you have ride height where you want it, you may only have an inch of travel to coil bind. With our 14x2.5x80 you will have about 2.9" of up travel to bind, with our 14x2.5x100 you will have 5" of up travel.
You go to a 12" spring I wouldn't go any lower than a 90 rate, which will give you 2.5" to bind of up travel.
Depending on how much stroke you have will define how much up travel you will need.