You have to know the aspect ratio of your phone's screen. It might be for example called "18:9" which boils down to 2:1 and you have to define if you want to view it in panorama (upright/vertical) or landscape (horizontal).
Basically you look up your phone's display resolution and just divide the horizontal value by the vertical value.
Your source video is 16:9 (1.777:1), which means, in terms of the upper example, that you will have to crop it somehow to get it full-screen on your phone. But first of all you have to set up your project to match somehow your desired aspect ratio. In terms of quality the best way would be to go not only for the aspect ratio of your phone screen but as well for the native resolution which could be 2880x1440 for our example.
So one way would be, to set your timeline resolution to 2880x1440 and throw in your clip, scale (and crop) it, and export it in timeline resolution out of resolve.
The drawback for this is that your computer would have to handle this high resolution while its not really necessary because your clip is much smaller.
To avoid this you could just go for the correct aspect ratio and stay resolution-wise close to your source footage. In this case, the scaling would be handled by your phone hardware when you go to full-screen mode while you keep your file small and bandwidth usage low.
You could for example set your timeline-resolution to 1280x640, this crops your source and gives it a 2:1 aspect ratio. If you watch this on your phone it should use the whole display when playing it full-screen.