According to https://ionicframework.com/docs/cli/cordova/resources/, icon source image should be at least 1024x1024px, and splash should be 2732×2732px. This seems also to be required when generating resources for iOS.
Also add pre-scaled iOS resources - generating them via ionic cordova resources ios requires an Ionic account, which might discourage potential contributors.
According to https://ionicframework.com/docs/cli/cordova/resources/, icon source image should be at least 1024x1024px, and splash should be 2732×2732px. This seems also to be required when generating resources for iOS.