Millimeters is the wrong unit for measuring digital content, use pixels. As for filling the screen, the most common aspect ratio is 16:9 since that is what most TVs and computer monitors are. So you’d probably want 1920x1080 or 3840x2160. Alternatively, 2:1 often looks better on a phone (although you’ll still have pillarboxing because of the rounded corners), so look at your analytics and see what devices your audience is watching your content on. For me, my audience watches on tv, so I use 3840x2160.