At present, you cannot decrease the size of a disc. We have purposefully disabled this functionality as it can lead to filesystem corruption and data loss.
The UI for editing a disk says
Resize Root Disc “disc-1”: (Discs cannot be made smaller)
But when sizing a new disk there’s no mention it can’t be shrunk in future. Nor does https://panel.bytemark.co.uk/servers/add-server when creating a new VM warn that they can’t be shrunk. Can you please consider updating all the bits of disk-size UI to have the warning so the initial size is considered more carefully. Otherwise it’s easy to think I’ll go for extra now and shrink later when I see what’s needed.
Is there a plan to allow shrinkage in future? I realise you’ll have users that would give back disk still being used by the filesystem, but perhaps the panel can look at the partition table to see it’s unallocated? Presumably, gparted on the existing rescue image supports shrinking the filesystem today, so it’s disk shrinkage that’s the issue? And you could always only enable it after it’s requested from support@.