Terrain Party to Unreal 4

So i am able to export from terrain party a section of map and tile it and import it into Ue4 for world conposition. But it seems the map is only a certain size , like a 1081 x 1081 or something, so i am wondering if i can get it to be bigger ? I mean in terrain party it is like 18km copied , but don’t seem that big in Unreal. Is this size the way it is big enough for an open world game for most people? What if say one wanted a map twice this size , can it be done ? Also the tiling code i am using , if set to 256 vertices per square , it tiles it 4 x 4 , so 16 tiles. Is that not enough tiles for this size ? thanks for any help