How to use these sizes
Design your screenshots once at the largest size, 1320 × 2868 for iPhone and 2064 × 2752 for iPad, and export a 6.3″ set (1206 × 2622) as well. App Store Connect scales them for older devices, so you rarely need more. If your layout looks different on smaller phones, upload those sizes too.
On Google Play, 1080 × 1920 portrait (or 1920 × 1080 landscape) is the safe choice: it is valid on every device type and meets the size Google asks for to show your app in recommendations.
What makes screenshots convert
Most people decide from the first two or three screenshots, often without reading the description. Show the main benefit first, use short captions, and test variations: Apple has product page optimization and custom product pages, Google Play has store listing experiments. More tips in App Store screenshots and app store optimization.
Once the new screenshots are live, your reviews tell you whether the app keeps what they promise. allstarsbot posts every new App Store and Google Play review to Slack or Microsoft Teams.