The rods will need to be trimmed if you bought 12" length. My guess is the rod length will need to be around 10" overall. The end links don't have to be 12" exactly, especially if you lowered your car. The MSP ones might even be too long, but Mazda figured they could get away with the same end links in the front because the struts were set up the same.
1. Go to your car, unhook stock end links, set sway up to be parallel with the ground (while your car is on the ground), and measure from the sway bar to the strut mount. That's your overall length.
2. Thread the rod into your rod end, mark the rod end, unthread it, measure the threaded length (this length should also be available on McMasters site, noted by "A" in the specs).
3. Cut the rod to the correct length, which will be:
Overall Length = (Rod end length - thread length) + rod + (Rod end length - thread length)
For example:
Using this rod end:
http://www.mcmaster.com/#6072k333/=3mmvbv
A = 1.1875"
E = 1.625"
Difference = .4375"
Difference * 2 = .875"
So, your rod end length for the entire piece is .875"
If my desired length is 12" from sway bar hole to mount hole, I need to cut my rod to 11.125".