Unfortunately I can't tell you for sure how much disk space you will need. This depends on the snapshot chain. What you may do is to "Delete" the snapshots one-by-one (always the one closest to the base disk, i.e. the oldest snapshot) and check the disk space after this. If a snapshot is left over in the datastore after all snapshots are gone from the Snapshot Manager, create a new snapshot and immediately run "Delete All".
André