Yes you are correct that both components are necessary for the correct archival and restoration of a project (unless using an export qcp file in which case that qcp file contains them both within when creating the qcp export).
The DB component is mainly linkages and dynamic/virtual data and very small in comparison to the repository which contains the actual physical files and information. For MS SQL server, finding the size of the DB component is pretty much the same procedure as finding the repository files you said you already know how to find. You would need something to access the DB such as SQL Server Management Studio. After logging into the access tool using the right credentials, you would expand the Database folder and then find the project DB. it generally uses the naming construct of <domain of project>_<project name>_db as the default naming construct. If you use a different naming construct then you would need to access the DBID.xml file for the project (it is within the repository location for the project files) and find the <DB_NAME> xml header and the PROJECT DB NAME will be between those designators. Select this project DB name within your SQL instance connected to and right click and select the properties as you did before with the repository files.
Hope this helps.