Note: this article appears to be talking about Nano Server as an a traditional OS. Microsoft has abandoned[1] that approach and is instead only offering Nano Server as a container image[2]. Compared to more conventional Windows images whose size are measured in gigabytes, the Nano Server docker images take ~120MB to pull and ~300MB of disk space.
[1]: https://docs.microsoft.com/en-us/windows-server/get-started/...
[2]: https://hub.docker.com/_/microsoft-windows-nanoserver