How big is this app update? Is it possible to see the size of the download from the App store when updating an iOS device?

As some users have a limited amount of online data they can use in contracts, It would be useful to know the size of the download. Currently the only way I can find out is if the update is larger than 20mb. This then tells me I have to update over a WiFi network.
The size is normally the size of the full application. What I do is I open the application info screen and look at the file size. Remember, App Store updates are full versions of the file being downloaded, and the existing one replaced.

There is unfortunately no way to do this quickly, you have to check each application individually from the update screen by tapping on its icon.

As pointed out by @cksum, when finding the app on the iPhone, scroll to the bottom and it will show the size.