After archiving an app version, on the organizer window there was a button, clicking it calculates and displays the estimated size of the app on the store.
In Xcode
The best way to get app size for all kinds of devices and models after app thinning is to look at the App Thinning Size Report, generated by Xcode. This report is generated when an Ad Hoc build is exported from an Xcode archive file.
Open Window -> Organizer, select the archive and click on Export button.
Choose method of distribution: Ad Hoc; app thinning: All compatible device variants; leave Rebuild from bitcode checked.
Proceed to generate the ipa file. Find the report alongside the generated ipa file titled "App Thinning Size Report.txt". It looks like this:
App Thinning Size Report for All Variants of MyAppName
Variant: MyAppName-iPad (4th generation)-etc.ipa
Supported devices: iPad (3rd generation) and iPad (4th generation)
App + On Demand Resources size: 50.4 MB compressed, 109.5 MB uncompressed
App size: 28.6 MB compressed, 87.7 MB uncompressed
On Demand Resources size: 21.8 MB compressed, 21.8 MB uncompressedVariant: MyAppName-iPad Pro (9.7-inch)-etc.ipa
Supported devices: iPad Pro (10.5-inch), iPad Pro (12.9-inch) (2nd generation), and iPad Pro (9.7-inch)
App + On Demand Resources size: 46.6 MB compressed, 110 MB uncompressed
App size: 24.8 MB compressed, 98.2 MB uncompressed
On Demand Resources size: 21.8 MB compressed, 21.8 MB uncompressedVariant: MyAppName-iPhone 8-etc.ipa
Supported devices: iPhone 7 and iPhone 8
App + On Demand Resources size: 45.9 MB compressed, 109.3 MB uncompressed
App size: 24.4 MB compressed, 87.9 MB uncompressed
On Demand Resources size: 21.4 MB compressed, 21.4 MB uncompressed...