The FFVII remake is episodic, Square Enix aren't sure how many episodes there will be. Each episode will be priced as a full game, which is to say $60 base, or $80+ for the version they expect you to buy.
The first episode will be on two discs. The Playstation 4 uses dual-layer Blu-Rays, so the episode needing two of them means it's more than 50GB in size. Probably significantly more, or else they'd max out one disc and put the remaining 5-10GB in a mandatory launch day patch.
Being a modern AAA title, there will probably be a mandatory launch day patch anyway, mind you.