https://developer.wordpress.org/cli/commands/db/size/#options

Currently the options are
options:
– b (bytes)
– kb (kilobytes)
– mb (megabytes)

https://github.com/wp-cli/db-command/blob/master/src/DB_Command.php#L716

Confirms that neither gb (gigabyte) or tb (terabyte) are supported as options.