They will be stored in the apps own file space.
If you are running iOS 5, you can go to Settings > General > Usage. If the TED app is not showing choose Show all Apps. Tap on the app to get a breakdown of how much it is storing in it's document space.
If you are not running iOS 5, let me know. There is a way to use iTunes on the computer to estimate this; but it a pain to do, and I don't feel like typing out the instructions unless they are actually needed.