I have four of these disks in operation. Top-notch hard disks. Whether a double-sized disk makes sense depends on the raid system. I have formatted the NAS with "Synology Hybrid Raid". With this, a hard disk can fail and no data is lost. But this also means: If I were to insert a fifth 8 TB hard disk in addition to the four times 4 TB hard disks, only 4 TB of this 8 TB hard disk would be used, the rest is "lost". Here you can look up which hard disks make sense for which Raid configuration:
https://www.synology.com/de-de...