FUNCTION12 supports Figma screens designed only on a Frame
Frame is a Figma feature that is normally used to create a container of your desired viewport size for UI design. More details on Frame can be found in this link.
Best Practices for designing a Page/Screen
Use Frame Tool or Figma Presets
Create a screen on Figma canvas with the Frame tool or choose your relevant viewport from the Figma presets from Properties Panel, as shown below.
Height and Width
Keep the height and width same across all pages & screens, except when the page / screen includes overflow behaviors like a vertical scroll.