Yeah, I'm learning this the hard way. It was recommended to me to get the 12" length springs. But my issue was that if I have the lower spring perch set down low (to get the car as low as I wanted), I have rubbing issues with the inside of the tire and the spring that would require me to run an even thinner width wheel or run spacers, no go for me on either. This is on a fox, not sure if your year vehicle will have the same issues.
So thinking about my specific situation, I need to set the lower spring perch high enough so it clears the tire. Then I need to figure out how much suspension bump/compression travel I want, really how low I want the car to sit for aesthetics but high enough to prevent the car from hitting the bump stops in street/autocross/road course conditions (read usable suspension travel). And of course will need to make sure the spring doesn't coil bind either. So as you can see, there are a number of conflicting things to consider when selecting spring length. If you don't have any clearance issues with your tires and can put the lower spring perch anywhere, 10" would be best (most amount of available suspension travel).
For my case this year, season was ending fast and I needed to get the car out. The 12" hypercoils put the car up like a truck so after taking a million measurements, fighting the internal battles within, purchased some 8" summit racing 300lb/in springs. Ride height is where I intended it to be (LOW) but for handling purposes, will be going to 9 or 10" springs for more suspension travel this winter when the torque arm goes in. So if you know anyone looking for Hypercoil 12" 270lb/in (like <2miles on them) or summit racing 8" 300lb/in (both 2.5" dia) I got some for sale, LOL.