I know Esenthel supports unlimited sized terrains, but is it possible to actually see far in esenthel?

Like if i was standing on a mountain, maybe 1km up and looking around me.

Is that possible or do still have to create huge levels?

D.viewrange(x);

But keep in mind that as much as you increase the distance more resources will require the game.

Thats what i meant, is there a way too see far efficiently? Like in the Operation Flashpoint games? Or in ArmA?

For example you can use models with various LOD or billboards (for plants and trees).

Design your level efficiently as well. The engine will only render what you can see. If you place some high mountains, it will not render what's behind there. If you make a huge, flat world, filled with objects and lights.. That might get a bit heavy on your performance if your viewrange is high.

It does render things behind terrain, it is only preventing things outside of viewrange and view field(camera view).
If the objects had been hidden/occluded it would've been visible in wireframe(eg occlusion culling) this is also the fact with interiors in EE atm that it is basically rendering everything in the view range even throughout 1000x rooms across, visible or not to the camera.

Not exactly. Models will be rendered but they will be occluded by depth buffer. So pixel shaders won't get executed