Welcome! 
As far as I know when you use the world composition you can create terrains which are larger than the size -> so theoretically you can create a 100x100 mile map, but you also have to think about the performance… 
"World Composition relies on a world origin shifting feature which, when used with distance based level streaming, allows you to create worlds which are not limited to the WORLD_MAX constant value hard-coded into the engine. "
https://docs.unrealengine/latest/INT/Engine/LevelStreaming/WorldBrowser/index.html