Here are the steps:
- Set Settings > Video Mode to 720x1280.
- Add your video.
If the video resolution of the clip is less than the video mode resolution, Shotcut will scale it up to fit. - Click on the clip, and add the Size, Position & Rotate Video Filter
- Adjust the size of the clip to be whatever you want. If you want exactly 480x848, you can type that into the filter parameters in the Filters panel.