In order to find the file sizing for the repository you will need to know where that repository is located (either local to the ALM server or remotely on a network access drive), and once you know where that is then you will need to have permissions to access and view that location. In order to find the repository sizing, and location, log into ALM Site administration, go to the Projects tab and select the project desired to know on the left side. Then on the right side find the Physical Directory path location for that project, and then go to that location within a File Explorer window. Once you are at that path location, right click on the main project folder, select "properties" in the pop up window and then a window will open and report the size of the repository files associated with that project.
If you choose that same repository location and perform the same steps on the parent "qc" folder to the projects folders you will get the total repository file size (assuming you dont have what is called a fragmented repository where it is located spread over different drives and locations...a completely non standard and rare implementation but would be an exception and would require doing this for all location and adding the storage amounts to get the total).
Hope this helps,