To be honest, I strongly disagree with XD's 1920 default. It's too large for designing for the average desktop. It creates problems later when a client or developer tries to view the prototype, and can't view it at regular size (without scrolling) because they'd need a monitor that's even larger than 1920 to fit their menubar, web browser interface, and the design.
If you look at website monitor stats (like these) you'll see that 1920x1080 makes up only about 22% of people. The majority of the rest have smaller monitors. For designing for a desktop/laptop montitor, I recommend 1440 at the largest, or 1280 at the smallest. I think 1440 is a good size. That's what I design with and typically recommend the people I teach XD to (I teach XD at Noble Desktop in New York City and online).
I would love to see Adobe add some smaller desktop sizes (like 1440, 1366, and 1280) and make 1440 the default.
If you already started your design, you can change your artboard size to 1440 (and set the Viewport Height to 900, which affects where the scrolling starts).
Best wishes on getting started with XD. I hope you enjoy designing with it!