The Call of Duty: Ghosts single-player campaign requires a 3GB install on Xbox 360, Activision has revealed.
In a post on the One of Swords blog, Activision community manager Dan Amrich confirmed that Ghosts will play on an Xbox 360 with an internal 4GB flash drive. Any compatible USB 2.0 flash drive formatted for Xbox with enough free space will work also, including thumb drives.
However, if you just want to play multiplayer, you don't need to install anything. As Amrich points out, the map Free Fall, which is a pre-order bonus, is an additional download, so you'll need around 250MB for that.
Meanwhile, the Xbox Marketplace has revealed that the Xbox 360 version - and presumably all current-gen versions - of Ghosts supports a maximum of 12 players in online multiplayer. This is down from the maximum of 18 in previous current-gen Call of Duty games.
It is expected that the PC, PlayStation 4 and Xbox One versions will allow for 18 players online.
While we're on the topic, on Xbox 360, Ghosts supports 1-4 offline players and 1-2 offline co-op players.