From my experience, the netapp cli command "vol size" returns size in the units in which size was set. In other words, if I enter "vol size vol1 1200g" and then enter the command "vol size vol1," I will expect a result of "1200g".
The command "df vol1" would return size in kibibytes (KiB) or 1024 bytes. The commandled "Get-NaVolSize vol1" command returns the size in bytes.
For example, here are the outputs for various commands in my environment for a volume named "vol1" (the volume name and NetApp names have been changed). The volume size was set previously to 200g by using the command "vol size vol1 200g"
filer> vol size vol1
vol size: Flexible volume 'vol1' has size 200g.
filer> df vol1
Filesystem kbytes used avail capacity Mounted on
/vol/vol1/ 188743680 80478696 108264984 43% /vol/vol1/
/vol/vol1/.snapshot 20971520 373032 20598488 2% /vol/vol1/.snapshot
PS> $volSize = Get-NaVolSize vol1
PS> $volSize.VolumeSize
214748364800
PS> $volSize.VolumeSize / 1GB
200
PowerShell allows conversion from bytes to Gibibytes (1 GiB = 1024^3 bytes) by dividing the bytes by 1GB.
When using Set-NaVolSize, the size is specified in the same format used by the NetApp CLI. So, a NewSize value of "214748364800" is just as valid as "200g"
I hope that helps.
Bill