Max terrain size 51.2 x 51.2 km?

[genoli]

I can't seem to be able to make terrains bigger than that?

Bug or hard limit?

[Bryan]

That would be limitations. 
Unity can't handle large world creation even with world streaming. 
You could try to enable world streaming but I would do so while your task manager is up. 
This size of the world seems like you would be hitting Unity's memory allocation. 
Potentially your PC specs as well. 

We do have a solution coming for things such as this large of a world. 
We showcased it a little bit in our last Live QA session. 
Storm would be needed for something this large.