The best way for terrain tile size for streaming (huge map) 100 km square

[Bavaria]

I will make a 100 km square I don't know to best terrain tile size for optimize terrain streaming. thank you

[Bryan]

I would do something like this: 
A 10x by 10z 

Tile size depends on you. 512 would be optimal. 
I would create terrain scenes and unload terrain scenes. 
Add floating point fix- due to the size of the world (so you dont have any weird texturing issues).