I don't know what open world means any more.Some people think that open world means completely seamless areas (i.e not loading) which is just a technical aspect in my view.
When I say open world,I mean a game where the player mostly steers the game's progression as they desire,regardless of whether there's loading or not.
Lightning returns,for example, is pretty much the only FF game where you can progress to the end game in almost any order or fashion you like.
I don't think FFXV will have non linear progression like that because of the story ,but I think it will end up being the single largest FF world and probably towards the end you'll get to do whatever you want or travel any where in the world.
I think FFXV will probably have the best world in the series and the most exploration ,but the story will surely be linear(which is important to have a good story)
EDIT :For all intents and purposes,it will be a big,large world with tons of exploration."Open world" labels be damned.