- You can undock the viewport from the editor and change its size but it will be on a separate editor window.
- To maintain the viewport inside the main editor you’ll need to dock other tabs to the side of the viewport (like the outliner and details panel) and resize those until the viewport has the size you need.
both of this options have the downside that the viewports buttons and menus don’t have resolution awareness.
so i think the other options are:
3. Add a second viewport next to the main viewport with your desire resolution.
4. or maybe add a camera with your mobile settings and use that in the second viewport.
hope that helps