It's not the size that matters, it's how it is used.

I was thinking that it comes down to how densely packed the content is, but then I remembered Shadow of the Colossus. There the world was mostly empty and devoid of anything to do, yet it felt perfect for the game and created the right atmosphere. I really think it depends on the game and what suits it.

Depends, but usually, I'm not bored with large worlds because though there might be a lot of running, it's fun to explore

It's not a matter of which is best, it's a matter of which suits a particular game.

I prefer more linear games over open world games, but the world size doesn't matter too much it's how they do it, and how much detail they put into it.

I don't see anyone could say The Witcher 3 felt empty. That game was full of stuff to do, even in the most obscure places. It's the best open world game I've played.

I like big and small worlds. Big worlds like WoW I love for the sense of travel and the scale of everything within that world. I love how I can travel from Kalimdor to Northrend via Blimp and then choose to fly, walk or ride a mount around any of the continents (besides Draenor which banned flying for most of the content). I don't know if Fable II counts as a small world (it does have loading screens and most areas aren't all massive and vast) but it sure feels cosy and full of life in some areas, though I feel for smaller worlds that over time things can become more or less the same, in a way that it takes me less times to see the same building or town because the world is that much smaller, even with better quality, time can still make a world that small dull and static.

I'd say the one biggest issue I have with most open/small world games is the fact that you can do many things in them but the AI don't exactly evolve as much within that world or notice all the deeds you've done as much (Skyrim and other big open world RPG's tend to feel forced or dull when it comes to noticing the fact you just saved the world and everyone in it).

Recently for me in terms of OW has to be Dying Light. Now I do love it;s visuals and it;s location, however what I don't like is how static the world itself feels in terms of most houses/apartments being incidental copies of one another. With the new expansion that just came out I managed to come across 4 houses that had the same items, same objects and layouts, just mirrored differently and honestly that broke a few levels of immersion. It also doesn't help that the majority of AI don't really interact with you in that world unless it's mission based, Zombies however will attack you from the moment you enter their line of sight, go out of it and they resume typical programming and won't really try tracking you down a long way or for a long time. The missions can also get repetitive over time with "fetch this for me" or "Kill X for me", I've seen way too many RPG's do this and it all just becomes irksome white noise and loses my interest in a much shorter time than say a few years. I do like Dying Light though, I just think most of it;s world is "dead" in terms of making you think it's alive and responsive when it's just run purely by specific triggers.

I'll let you know when (Linear) Persona 5 and (Biggest Open World to Date) Final Fantasy XV come out.

This question is just way too simple, and there is no right answer the way you've phrased it.

I like open world games as long as they don't feel empty and there is always some objective or mission to do.

Right now we have this "Open World" obsession going on. Some games that adopt it will continue to use the format and some games will try it a few times before they move on to something else. It's widely used but I can't help but think it's a fad that'll end up overshadowed by a different style. I will take small world or large world games. It depends on how well the developers make the game.