Your disks also don't need to be the same size. Right now, I have a 3.6 TB disk, and two 7.3 TB disks. This makes for 10.9 TB actual available space, as it uses some of that disk space for redundancy, and some of it for expansion.
You can also gradually grow the SHR later on. I could eventually replace the 3.6 one with a third 7.3 TB disk, or a larger one.