You actually need more than two commands to get all of the info you are looking for:
Get-NaVol will return the volume size and most properties about it. Get-NaVolsize works too, and is more specific.
Get-NaSnapshotReserve gets the reserve amount.
To get the closest output to DF -HR, try this:
Get-NaVol | FT -autosize Name,@{ "l" = "SizeGB"; "e" = { "{0:#,##0}" -f ($_.SizeTotal / 1gb) } },@{ "l" = "SizeUsedGB"; "e" = { "{0:#,##0}" -f ($_.SizeUsed / 1gb) } },@{ "l" = "SizeAvailGB"; "e" = { "{0:#,##0}" -f ($_.SizeAvailable / 1gb) } },@{ "l" = "SizeReservedGB"; "e" = { "{0:#,##0}" -f (($_ | Get-NaSnapshotReserve).Size / 1gb) } }