How to create unlimited flat world?

How does your game-design want it? What should happen with it?

Technically, an unlimited world requires unlimited storage, which I assume most people do not have... Although the engine allows it, there are some options that you might wish to consider:

1. just build a large world in the world editor and use some teleportation to give the impression of a round world.
2. create heightmaps and objects on it through code, and delete it again after the player has walked through it.
3. Don't create an unlimited world, for it requires the player to have unlimited time to walk through, which gets kinda boring...
4. Work on game-code first before making a big-ass world

I want create map with no object etc. Only flat world.

Differently: how to create flat world with size: 50x50 using c++?

with C++ you can do it only for "Game" version of the world (not "Edit" as the World Editor does)
use Game::WorldManager::New, use WORLD_MANUAL for world load, and manually load and edit areas.
for full codes you can lookup Ineisis

if you want only a flat world without objects, perhaps you could just animate your character on the spot. I mean have a movement animation, but don't actually change the position of your character.

Just an idea, of course it depends on what you want to achieve.