diskVolumeReports.volumeInfo.0.storageTotal should show total storage, and, after you reformat it to GB, be the total storage of the device.
diskVolumeReports.volumeInfo.0.storageFree should show how much of that is free.
However, some devices seem report completely wrong numbers.
About 4k of my 5k devices report logical numbers in the 0 columns, but the last 1k either report weird numbers, or nothing at all.
For many, they instead report the relevant numbers in the volumeInfo.1 columns, but off by a factor of 1000.
So perhaps listing kBs instead of Bytes.