Project Natal needs 13 feet of free space

Natal requires you to have thirteen feet of free space to play in your lounge
Natal requires you to have thirteen feet of free space to play in your lounge

Microsoft has revealed that you will need 13 feet of free space in your lounge to properly play Project Natal games.

That's four metres. Of space in your lounge. Not taking into account sofas and coffee tables and bookcases and chairs and other encumberants that might get in the way of you pretending to play a virtual game of tennis on your TV.