I actually don't think this is possible with Resolve 14 because I don't think Resolve 14 is able to deliver cropped video. However, if I use HandBrake to crop the video, the encoded file is the correct size and fills the phone screen because HandBrake does set the video file to the cropped video size. The only problem is that I then need to use Resolve to edit the clips and HandBrake to crop the video which is two lots of encoding. Is there another video editor which has basic editing facilities (timeline with cutting option mostly) that also supports export cropped video?

Thanks for your answer. I have been trying to do as you said. So I create a new project and add my clip (1280 x 720) to the timeline. Because the clip I'm using is from OBS of an Android camera, I only want the left quarter width of the clip so I crop right, and a little crop top and bottom to match the actual video in the clip. Then I go into project settings and set the timeline resolution to 640 x 1280. But once I do that, the video in my timeline is now really small. The black video background is right size but the clip in it is tiny. How do I make it fill the background?