here is the command i use (very simple) :
Get-NcVolSize <volume name> -VserverContext <vservername>
when the volume is under or equal to 1024 (return in json format) :
"VolumeSize": 548682072064,
"IsReadOnlyFlexVol": false,
"IsReplicaFlexVol": false,
"IsFixedSizeFlexVol": false
when the volume is above 1024 (here set to 1025) (return in json format) :
"VolumeSize": 0,
"IsReadOnlyFlexVol": false,
"IsReplicaFlexVol": false,
"IsFixedSizeFlexVol": false