Question about open world game

How would I create a very large island where I can add other islands and have a rigged flying car?

What’s very large - how does it translate into human units?

That’s a very general question, you need to break it down into more basic tasks and then search for information on how to achieve those things.

If say I were to make a map 4 times bigger then fortnites map. How big should a cell be?