Stock shackles are 4" between eyes.
Shackles will lift approximately 1/2 of the length that they are longer than stock. For example a 5" shackle will lift 1/2", a 6" shackle will lift 1", and so on.
I would not get anything longer than a 1" lift. Lifted shackles will add caster to your steering and the longer they are, the worse it will be. I tried a 2" lift shackle when I first got my Jeep and it was pretty bad. I now have 6" shackles (1/2" lift) on it and I have no problems whatsoever.
If you want to fit 31" tires on your Jeep, a 1" lift shackle should be ok. Anything bigger than that and you should do either a suspension or body lift.