Hi Richard,
well there's the commands df-Ah (df -Vh) and aggr show_space 'aggr name' to give you some information
on the space used/available on the filers.
In is not possible to give you an command that tells you how large an aggregate, volume and lun are possible. Reason
for this is that there are to many variables to take into account: Do you want snap reserve? Is if a volume with or without al lun? thin? thick? etc.etc.
Same goes for the aggregate: do you want raid_dp or raid4? how many spares? etc.
If you are not sure what the best sizing is for the volumes/lun you want to configure you can allays check with your support.