Referring to this:
Say there's 3 major islands, id love to see 1 disc per island. Jam pack it full of amazingness and what not. Plus another disc separate for multiplayer.
If the story is rumored to take 25 hours there could also be the need for an extra disc considering how much dialogue there is, the amount of radio stations and how long they go for before repeating etc.
2-3 discs would be a winner. Just so you can tell its gonna be huge without even playing it yet.