A graphic that only spans part of a List looks tentative. A graphic that runs the full width of the screen looks intentional. The trick is knowing the aspect ratio that fills the List row edge to edge.
For full width List cards in Subsplash, design at a 16:9 aspect ratio. That fills the horizontal edge of the app screen with no gutter and no letterboxing. If you go taller or wider, the card either crops your graphic or leaves empty space beside it.
Design with a safe area. Keep any text or logo at least 10 percent inside from each edge. Different phone models introduce slightly different clipping behavior, and text that lives right against the edge is the first thing to get cut.
Resolution matters less than proportion. A 1920 by 1080 graphic and a 1280 by 720 graphic both fill the card cleanly at 16:9. Go higher only if you need the extra sharpness for large phones or tablets. Below 1280 wide starts to look soft.
Always preview inside a real List on a real device before publishing. The desktop dashboard preview is close but not exact. Ten seconds on your phone catches problems the desktop hides.