`tahoe backup` could easily report size information about the backup performed

[exarkun]

As a backup is progressing, information about how much data is being backed up is readily available. There are a few different sizes which can be considered:

• size of the local files being backed up
• size of the files which don't already exist in a backup and need to be stored
• size of all backup versions of this data set

Application-data-size vs Tahoe-LAFS-storage-commitment provides another dimension to this data, as well.

It would be great if tahoe backup could report some or all of these sizes.

Some of this information could also factor into improved progress reporting (reporting by bytes transferred may provide a more accurate indication of progress than reporting by number of files transferred).