Hi.
This is my first post in this community, and I hope that this is the right place to ask it, so here goes. I do apologize in advance if it’s the wrong place.
I’ve seen it answered earlier but it doesn’t seem like any of the previous posts (that I’ve seen) had a clear answer.
So, what’s really the size limit for an outdoors world for Unreal Engine 4? Could I reasonably create a 100x100 mile map?
Thanks in advance
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
The default WORLD_MAX is 20,000 * 20,000 kilometres I believe. This is the safe limit for a world before floating point errors begin causing problems with physics and general accuracy but it can be overridden in the source code. For a single player game you can use World Composition and origin shifting as suggested but origin shifting doesn’t work in multiplayer.
I’d personally like some clarification on the multiplayer impact of using World Composition without origin shifting and how this is handled by the server. I havn’t seen very clear information about taking this approach.
Alright, thanks!