Drop a creative in and see it exactly as it lands, cropped and chromed for the placement you pick. Stories and Reels carry true overlay safe zones, tighter on paid than organic. Feed has no overlay; a 4:5 asset shows a small guide for where it trims when rendered square. Turn on Measure to check text at true phone size.
Hold a real credit or ID card, any standard bank card, 85.6 mm wide, against the screen and drag until the bar matches its width exactly. This makes real phone size mode show the ad at true physical scale. The text readings work without this, but calibration makes the preview physically accurate too.