Frankly I’m a bit surprised this works …
$ guestfish
Welcome to guestfish, the libguestfs filesystem interactive shell for
editing virtual machine filesystems.
Type: 'help' for help with commands
'quit' to quit the shell
><fs> sparse /mnt/tmp/test/test.img 1P
><fs> run
><fs> blockdev-getsize64 /dev/vda
1125899906842624
$ guestfish
Welcome to guestfish, the libguestfs filesystem interactive shell for
editing virtual machine filesystems.
Type: 'help' for help with commands
'quit' to quit the shell
><fs> sparse /mnt/tmp/test/test.img 1E
><fs> run
><fs> blockdev-getsize64 /dev/vda
1152921504606846976

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