Installed Height is a spring spec, not a valve length spec.
When you buy a set of springs, they are designed to be installed at a certain height. This is the proper height of the spring that will give the correct amount of seat pressure, and is controlled by the length of the valve stem. If the valve is longer than the rated installed height of the spring, there will not be enough seat pressure and your valves won't seal to the seats, causing major compression and valve float issues. If the valve is shorter than the rated installed height of the spring, the spring will apply too much seat pressure to the valve, which will cause the valve to beat the shit out of the seats. It can also cause a flat tappet cam to go flat by making it harder for the cam lobe to open the valve. On a roller cam, too much seat pressure can ruin the roller axles on the lifter, and cause a hydraulic lifter to blead down.
Other issues with having too short of a valve stem will be incorrect valve train geometry and excessive coil bind.
I'm pretty sure you find out your installed height of the valve with retainers first. You can't find out your installed height just from the springs, you need the valves and locks first. Then adjust the spring you're going to use accordingly