Comment 12 for bug 1845571

Reading a bit more code....

ubiquity has partman/check.d/partition_too_small which does read /cdrom/casper/filesystem.size (which is 5GiB) and should actually prevent from successfully contining to install on the installation media.

Also there is d-i/source/partman-auto/lib/auto-shared.sh get_auto_disks() which should be skipping /cdrom , ie.

                # Skip devices containing the installation medium
                [ -e "$dev/installation_medium" ] && continue

                # Skip installer disk
                $(mount | grep -qF "$device on /cdrom ") && continue

So we need to check that those pieces are working correctly and that devices smaller than 5GiB are never offered for installation *both* when they are and are *not* installation media.