How to check iOS app size before upload
This question is quite old, but IMO still relevant. Here is a recent message I received from Apple when I uploaded an app for review:
The app binary listed below was 79.7 MB when you submitted it, but will be 132.2 MB once processed for the App Store. This exceeds the cellular network download size limit and would require your app to be downloaded over Wi-Fi.
There is no way to know this 'once processed for the App Store' size until you've submitted your app.
There is a feature within the Organizer/Archives view that allows you to estimate your final app size. I have found this estimate is inaccurate. Here is the button you're looking for if you want to see this estimate:
So what I have learned is that the "processed for the App Store" part above will mean your .ipa file is not an accurate measure of the final app size users will download. The Organizer/Archives estimation is incorrect, it seems the estimation is higher than the actual size, for example an archive sent for review was estimated at ~109 MB, the final app size in the store was ~90 MB and the .ipa filesize was ~47 MB.
Why is the final size larger than the .ipa?
I'd also like to know the answer to this, from what I understand it's related to DRM things that Apple adds to the .ipa.
UPDATE (June 2015)
The latest version of Xcode does not have the 'Estimated App Store Size' option within the 'Organizer' view.
UPDATE (November 2017)
Estimated size information is available on App Store Connect:
My Apps > App > Activity > Select a build > App Store File Sizes
The install file downloaded from the App Store will vary based on the device and what thinning is done to your app. An ad hoc distribution .ipa file will give an estimate of how big the install file from the App Store will be. Archive your app in Xcode, then make an ad hoc distribution. If you turn off app thinning while making this file you'll see the biggest your install file could be. If you turn on app thinning and generate all the variations of .ipa files you'll see all the different install file sizes.
To see an even more accurate answer to this question, submit your app to TestFlight. In the TestFlight app on your devices you can see the size of the app by checking the app details. Check it on multiple devices, it will vary if the app thinning process considers your devices sufficiently different and uses different assets from your app archive.
Below is my original answer:
In Xcode 4: Once your app is ready for distribution, archive the app. In the Organizer, select the Archives tab. Select your app in the left hand column. Select the latest archive in the right hand column. Hit the Share... button in the top section. Save the file as an .ipa. Check file size in the Finder.