How to change landscape size

I just want to hash out the basic landscape of my map. How do I change the map size? It needs to be about 25km2 can I do this on one level? I know I will need enhanced tools to create the full map with all the extras but just to create the basics on one map like the mountains and the canyons so I know when making the individual levels that will later be linked up how they fit into the big picture.

I have edited the scale and this created a large map but then all the textures are also increased to a stupid size that looks odd.

Ok, what I should have made clearer is that I want to create a rough map that I can divide up easier into sections that I will add in details like foliage to. But as far as I can work out the largest map I can create is about 5km the main city is about 3km2 and the mountain is 2km2 with a desert and canyon section as well. I mocked it up in the terrain editor for Ctyskylines but need to progress into Unreal engine.

Also if I’m going to make a ‘quilt’ map that is later joined up using one of the tools around, how do I match up the edges? I’ve built a few smaller parts of the map to showcase different areas but I’m estimating each of them is only around 1km at most. Maybe only 0.5km.

Just looking for some pointers that’s all. I have the story, progression, characters, all written up and ready. It’s just this initial push in creating the map that’s the biggest hump.

Map will not be the biggest hump!

That being said, look up “World Composition”, there are great videos made by Unreal on youtube about it.