If you’re new to game development – or even not at least a couple projects into your career – I don’t even stop at not recommending an open-world game: don’t do it. It’s just bad news and sadness and you realize how complicated worlds are and then you become a misanthrope and then there’s just no hope for you while you rabbithole down a pit of despair.
What I’d recommend, if you’re set on large worlds, is “large scale” worlds. Say, 4x4 Km (that’s what I’m doing – and it’s a whole hell of a lot of world to detail). You’ll probably still end up accomplishing the feel you’re going for and you have a much, much higher chance of success.