Unfortunately, macOS itself is pretty heavy and takes up 20ish GB itself. Tart can work with smaller VM sizes but it's hard to create one.
On a bright size Tart compresses the disk layers and you'll only need to download about 18 GB over the internet.
Can you run Linux under it?
Not at the moment. For the initial release we focused on macOS but the Virtualization.Framework supports Linux. I've created a feature request. Feel free to comment with your use case for Linux. https://github.com/cirruslabs/tart/issues/62