All new Xbox One comes with 500GB internal storage which is not removable and upgradable too. Microsoft also revealed that the support for external storage won’t be available at launch. From the reviews of Xbox One, each big next generation game after installation takes about 50GB of storage. So, basically you can install about 10 huge games at a time on your Xbox One. One of the glaring omissions on Xbox One OS seems to be the storage management. There is no overview page where you can look at the storage space remaining, which game is taking more storage, etc,. It seems Xbox One OS itself will do the storage management somehow.
Neowin spoke with Microsoft and got the following response,
Xbox One was designed to make storage management automatic. For saved games, settings, and other information that Xbox One customers save to the cloud, space is virtually unlimited. On the internal hard drive in each Xbox One, games and apps can be uninstalled or reinstalled instantly with the click of the Xbox One menu button. Saved games and settings information is retrieved from the cloud for any game as its being reinstalled. By being smart about how storage is managed, Xbox One keeps everyone playing, watching, and sharing their entertainment content rather than worry about limitations. You can also see how much storage any app uses by pressing the menu button on that app.