iOS

Create an Xcode App Size Report.

First, by configuring the app version and build as described in the
iOS create build archive instructions.

Then:

Run flutter build ipa --export-method development.

Run open build/ios/archive/*.xcarchive to open the archive in Xcode.

Click Distribute App.

Select a method of distribution. Development is the simplest if you don’t
intend to distribute the application.

In App Thinning, select ‘all compatible device variants’.

Select Strip Swift symbols.

Sign and export the IPA. The exported directory contains
App Thinning Size Report.txt with details about your projected
application size on different devices and versions of iOS.

The App Size Report for the default demo app in Flutter 1.17 shows: