That’s the max size of the unsigned integer 16 bit that is in the code of the engine. Until they change it to uint32 which has a much bigger max value, we are limited.
Check out my USurfaceMesh plugin on the marketplace. It can now apply the mesh to the landscape. Maybe it might offer you some more features. I recently made it so you can capture part of the landscape, erode it and then apply it back onto the landscape. 
Unfortunately, when trying to apply bigger brushes that affect many more vertices in the landscape, it just is going to take a long time to process that. Maybe that’s why they have a limit.