App Store Connect has strict screenshot dimensions per device family. Wrong sizes can cause rejection or poor presentation. For iPhone 6.7" display you need 1290×2796; for 6.5" use 1284×2778; for 5.5" use 1242×2208. iPad and Mac have their own required sizes.
Device families
Apple lists required screenshot sizes for each device class. You don't need every size for every app—only the device families you support. Using a screenshot resizer that outputs all current required sizes ensures you're always compliant.
Tips
Use PNG or JPEG. Avoid important content at the very edges. You can upload multiple screenshots and get a ZIP with every required dimension—saves time and avoids mistakes.