Hi,
the backup task log will contain information about logical reused and total size for a backup.
If you are referring to the actual storage usage on the proxmox backup server, then this information is not available, as the physical used vs. logical size of a backup snapshot can significantly differ as the data chunks are deduplicated. Multiple snapshots can/will logically reference the same physically stored chunk.
The garbage collection will however show you this for the whole datastore, given also the deduplication ratio.