That’s a lot.
Far Cry 3 is a 6.2 GB digital download and in case you have bought a retail copy, guess what, it will take 6.2 GB to install the whole thing on your hard drive. (Via PSLS) Not as big as Gran Turismo 5, which was 10 GB in size, but not a lot of games have such installation sizes over 5 GB on the PS3.
Now this is not a big deal since you can put any hard drive on the PS3 since it’s not proprietary, but Sony released a 10 GB model of PS3, called the Super Slim, and based on that size you can install only one such game on the system.
Imagine having to uninstall Gran Turismo 5 to just play this game, that would be terrible because you need to download updates for GT5 again.
You can upgrade the PS3 hardrive even on the new 12 GB slim model, but you need a HDD mounting bracket for that, which Sony also sells.
Are you planning to buy the game on the PS3? Having played and finished the game on the system, I’d say it runs pretty well and our review was on the PS3 too, which got a perfect score.
Below is a shot from the box art which shows the install size: