Check this page: https://docs.mendix.com/howto/monitoring-troubleshooting/manage-app-performance

Most of these external tools when deployed together with Mendix, is capable of giving you more information (probably also disk space).

For one of my customer, I have developed a simple custom java action, that will return disk space. So, when we need to know the disk space that is available for that app, we can simply login in to that app and see that from a page, by executing a MF which calls the java action. See below