There isn't a way to make only the Writing window full screen quite yet, but if you click Writing from the top center, it will set the majority of the screen to your Writing editor. The left navigation panel will typically remain, depending on your screen size, along with the toolbar. You might also try zooming in (using CTRL + ) until the sidebar is automatically hidden.
We also plan on adding a Focus Mode in the future. Although I don't have an ETA on this feature yet, you can follow along with our updates and plans for the future on our Roadmap.