Indeed, you can start smaller, but most shops prefer to size their needs, install once, and tend to hardware when maintenance is required. If you are using virtualized systems that allow for dynamic disk space expansion (albeit with a little downtime), you can start smaller.
A default DAT-MAIL installs, along with the database server (Microsoft SQL Server), our default components (such as PostalWeb Desktop Dashboard (PWDD) and Windows 10 Professional together take up about 15 GB. You will need to consider paging file space for Windows, room for backups, Mail.dat files, and general data growth. In addition, your IT team needs to know that our products use Microsoft SQL Server.