A quick blog post to detail how you can check the Project DB size in Project Online. With Project Online, each Project Web App site collection can have a 10 GB Project database:
To check Project database usage for Project Online, access your SharePoint Admin page in Office 365. On the Site Collections page, select the PWA site collection, then click Settings from the Project Web App menu as shown below:
This will display the Project Database Usage:
Modern Work Management - Project and the Power Platform blog 