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

In this example, the app has an approximate
download size of 5.4 MB and an approximate
install size of 13.7 MB on an iPhone12,1 (Model ID / Hardware
number for iPhone 11)
and iPhone11,8 (iPhone XR) running iOS 13.0.

To measure an iOS app exactly,
you have to upload a release IPA to Apple’s
App Store Connect (instructions)
and obtain the size report from there.
IPAs are commonly larger than APKs as explained
in How big is the Flutter engine?, a
section in the Flutter FAQ.

Breaking down the size

Starting in Flutter version 1.22 and DevTools version 0.9.1,
a size analysis tool is included to help developers understand the breakdown
of the release build of their application.