I'm glad that you were able to resolve your issue and thank you for posting your solution so that others experiencing the same thing can easily reference this! Since the Microsoft Q&A community has a policy that "The question author cannot accept their own answer. They can only accept answers by others ", I'll repost your solution in case you'd like to "Accept " the answer.
Issue:
How to Retrieve Backup Item Size for a Specific VM in RSV?
Solution:
This link has the detailed explication for API that helps to resolve the issue https://learn.microsoft.com/en-us/rest/api/backup/job-details/get?view=rest-backup-2024-10-01&tabs=HTTP
If you have any other questions or are still running into more issues, please let me know. Thank you again for your time and patience throughout this issue.
Please remember to "Accept Answer" if any answer/reply helped, so that others in the community facing similar issues can easily find the solution.
https://learn.microsoft.com/en-us/answers/support/accept-answer#why-only-one-accepted-answer