[Rod P]

Originally Posted by David Pozzi

Each shock has a manufacturers recommended installed length/height.
Shocks with limited travel have less ability to vary ride height without topping out or bottoming, so I prefer a shock with 5.5" to 6" travel on the rear if possible. If I want to raise or lower the rear 1" the longer shocks will allow that. If buying a rear kit, the mfr will have a suggested shock length at ride height. Just make sure his design ride height is close to what you want to run your car at.
OR:
Set your car at it's desired ride height and pick a shock that fits that space. Assuming you have shock mounts in place to measure from.

Once you know the shock length at ride height you can pick a coil to fit it. Estimate the load on the coil and how much it will compress and pick a length that will still allow adjustment room up or down.

What he said!