The storage report in your Private Media Channel gives you information of the total storage used and a detailed report of the storage space used by each content. Here it provides information of the date the content was created, the author of the content, the community in which the content lies, and the author of the content.
You can search for the details by using the search filters in terms of date the content was created, community the content belongs to, title of the content, author of the content, or the respective author's email.
Once you have generated the report, click on the 'Export Report' button in the top right-hand corner of the page, and your report will be exported into an excel sheet.
There you go! Now you can determine the amount of storage space left in your channel easily.