A question we all need to find an answer to.
So does it matter if I make my world in one piece, or should I have it split in several smaller pieces and work my way around making it realistic?

For the moment I have a really big map (didn't check, but it's something like 300/200 "areas", with a big approximation, might be even bigger. All this is going to get populated by a ton of objects, items, trees, terrain stuff, river etc.

Will my game have a bad performance if I leave it as it is, or is the engine smart enough only to bother himself with the things in the visual range only?

It is about preference. What is really going to matter is how good the engine can handle LODing the terrain from distances. Also it will matter how far away you will allow your character to see out. Both are performance issues. I think doing levels instead of one massive world might allow you to cheat a little if you don't want to make a weather system. You could easily set up one level/world to look glummy....and then another one, make the world look hot without a cloud in the sky. Sometimes a project can get a little easier to manage if it is broken down in pieces. is your game on one continent or is about different continents? if many continents are being explored you might want to break that up in game pieces. If it happens all within the same area, I would not break it up...but that's just me. for performance...it will not matter. there are plenty of AAA games with the best quality game assets inside that do it both ways. Goodluck.

The objects update range is determined by D.viewRange(int range); and Game.World.activeRange(int range); The draw-or-not decision is based on that, plus a frustum check in the draw call. As long as you have those set you should be fine.
Keep in mind, though, that large objects will be drawn/updated/etc even when they really shouldn't be if a tiny piece of them passes all the checks. So the size of the world will matter less than the size of individual objects, if the 3 aforementioned things are set correctly.

I don't think your current design of one big world is going to cause you many problems inherently, as Esenthel is designed to support exactly this and by default is only ever going to have a small subset of those total areas loaded at any one time.

Performance I'd have thought is going to be more dependant on the poly density in areas (use at least one level of LODing preferably more), your use of lighting (watch the ranges) and shadows and viewport range.

Given a fairly uniform distribution I wouldn't expect performance over a huge world to be any different to that over a small number of areas. What I would do is a lot of testing initially over say a 16x16 populated area to get an idea of performance before rolling it out to the rest of your world.

Thanks a lot for the replies. I did know some of these things, but I still have a few questions: How about loading time? When the application starts, will the engine struggle to load all the thousands of objects all over the world in those containers, or just the ones in my range?

(My game IS on just one continent, and I would prefer to keep it in one piece. But I only worked on coding so far so I have no idea how it will perform with thousands of objects on my big island).

There's a nice explanation of the World Manager operation here