You can determine the space used by a FlexClone volume based on its nominal size and the amount of space it shares with the parent FlexVol volume.
When a FlexClone volume is created, it shares all of its data with its parent volume. Therefore, although the nominal size of the FlexVol volume is the same as its parent's size, it uses very little free space from the aggregate. The free space used by a newly-created FlexClone volume is approximately 0.5% of its nominal size. This space is used to store the FlexClone volume's metadata.
New data written to either the parent or the FlexClone volume is not shared between the volumes. The increase in the amount of new data that gets written to the FlexClone volume leads to an increase in the space the FlexClone volume requires from its containing aggregate.