When you upload screenshots to App Store Connect, you must use the exact dimensions for each device family. Wrong sizes can lead to rejection or automatic scaling that looks bad.
Required sizes
Apple publishes the current required screenshot dimensions (e.g. 1290×2796 for 6.7" display). You need at least one screenshot per device family you support. Use a resizer that outputs these exact dimensions so you never have to guess.
Order and count
Order matters: the first screenshot is often the one users see first. Provide 3–10 per device class. Resize all of them in one batch, then upload to App Store Connect in the Media Manager for your version.