You may wonder sometimes what was taking up all the space on your iPhone. Since you have a very limited space on your device, you may want to know how much space exactly an app is using so that you can manage them more efficiently, especially if you have a lot of apps. Here is how (iOS 5):
Choose Settings > General > Usage. There will be a list of how much memory each app is using. Tap on any app to find out the App Size in more detail.