This is what it says in the description online: "For the first time ever on a single Blu-Ray disc, experience Ezio's story in full in Assassin's Creed: Ezio's Story."
It will probably setup like any other game series that has multiple games on it, where you select the game you want to play from the main menu. You will have to go back to the XMB and restart to change games.
There is no info that I could find whether the DLC is on the disc or if you have to download it.
You can search the games on the PS Store, as the file sizes will be available there.
Assassin's Creed 2 and Brotherhood have to be installed into your PS3 (like AC1 has to be installed for those who got the limited editions of ACR). Assassin's Creed Revelations is the "Main" disc game of the collection. Meaning when you launch the disc, you would be playing ACR. to play AC2-B you need to have the disc in the system.
also I don't think they don't inlcuded the addons for the games, just the online pass for ACR. Although I think it contains the story memories 12 and 13 for AC2.
Overall Harddrive space, I believe you'd need about 12-15gb of space