For any type of extent, and also for ReFS, we tend to suggest 200TB as the biggest size. Even though some customer go even bigger where a physical server is involved, we recommend to not go too big to avoid potential failures (read also downtime, not just damages). Smaller server equal to smaller failure domains. This goes into combination with what written above, so if you need for example 600TB of usable space, then you know you need 3 extents. Sometime this number is bigger, since for example people using SAN as the source for their volumes can only create up to 64TB volumes, so this becomes the biggest size of a single extent. This is also true for virtual repositories when a VMDK is used,unless you want to use RDM (which we are fine with).