Which is the largest maximum size of land?

By default size of the world is limited to ~500k units (WORLD_MAX constant in sources). You can still place objects outside of these limits, however you will start experience unstable physics simulation on objects that are too far away from world origin. So it’s not recommended to place objects which simulate physics that far, but you can still place visuals (ex. skyboxes usually are much bigger than 500k). Right now engine will stop physics simulation for actors which are outside of world bounds(WORLD_MAX), you can disable this behavior in WorldSettings->Enable World Bounds Check.

It’s possible to create “infinite” seamless worlds. UE4 supports world origin rebasing. At certain moments (when camera position is far enough from current world origin) engine will shift world origin position to current camera position. Basically this operation subtracts a specific position offset value from all active actors in the world.
To make it work you will need to compose your world from a smaller streaming levels (tiles). Tiles should be loaded based on distance from the current camera. Each tile should have a “world position” which tells engine where that tile should be positioned in the world. While player travels trough world - engine compares tiles “world position” to a camera position and decides whether tile should be streamed in or streamed out.

We have a tool which helps you to manage big worlds. It’s hidden right now under Editor Preferences->Experimental->World Browser
If you will enable it there will be World browser tool in Window menu.
World browser managed worlds work bit different from single levels, you will need to open world using World Browser menu File->Open World…
“Open World…” command opens a level which will be your persistent level (level that is always loaded) and all maps found in the same folder and subfolders will act as a tiles.
By default tiles are not loaded and you need to double click on a tile name to load it to the editor.

World origin rebasing will not work with multiplayer games, to make it work you will need to implement your own server solution.

We are working on a sample demo which will show how to create a big worlds, also we have a documentation for this tool and it should be added soon.