I'm making a low-poly/flat shaded 3D open world game. But I need suggestion for how to make the world. Making it in seperate parts, one part etc. The world will become around 36 square kilometres and I'm using Blender

Thx in advance!

Please post your concept designs for the world so we can give a more accurate suggestion.

Here iT is[attachment=22580:image.jpg]

From your original question it seems that you're concerned about how to create -- how to model -- this environment in Blender.

Looking at your concept as reference, you can save time by modeling each island separately and limiting their lowest point to sea level (or ground level if that's the lowest you want to reach).
You can model the islands by using the sculpting mode with subdivided planes to rough out the volumes, and then manually tweak the mesh at the vertex level to get the proper shape and finish that it might be lacking.
Once you have all of your islands modeled the way you want, you can arrange them how you want them to be in the final map and focus on connecting them with a sea floor \ ground. This means extruding the outer edge loop of the islands downwards and scaling these loops outside, then joining the islands into the same object (CTRL+J) and welding the edge loop vertices as a sea floor \ ground to connect the islands.

If you're not technically literate in using Blender, you can find resources online:
- http://wiki.blender.org/index.php/Doc:2.6/Manual/Modeling/Meshes/Editing/Sculpt_Mode
- http://cgi.tutsplus.com/tutorials/secrets-to-creating-low-poly-illustrations-in-blender--cg-31770
- Blender 2.6: speed art of low poly scene

Your main focus now is producing a model that suits your needs.
Once your enviroment is ready, you can export it as a single mesh or generate a height map from it and import it as a terrain in most game engines.
The implementation details (model optimizations, splitting in sectors etc.) you can leave for later because they depend on the game engine that you'll use.