In the Google Play Store, when an app is too large in size, a "WiFi" tag is mentioned next to the app name. That way the user can know that the download is heavy. At what application size does Android add the WiFi tag to apps ?
Good question. I believe the answer has been recently changing, because while I was initially under the same impression as SaintWacko (thinking the cutoff was 25 MB) I have been unable to find any Google documentation defining such a limit.
What I have found is that the maximum APK (app) size is 50 MB. Developers can also choose to use up to two APK expansion files to be downloaded along with with the app's APK, each of which can be up to 2 GB. Even for these larger expansion files, though, I could find no mention of Wi-Fi only download restrictions (unless explicitly required by the developer). I remember seeing the Wi-Fi download flag you mention on some larger apps that I use personally, but today I've gone back to those same apps and don't see that flag.
As best I can tell this policy has changed.
While there is some conflicting information, the limit at which the market adds the WiFi tag appears to be 25MB.