tif files at that scale through the layout tool in ArcGIS Pro (400 dpi). I am sure there is a better way to do this, but currently I export snippets of each tile as. For my example map, I am aiming for a area about the size of western Europe, so I will be working at 10,000,000km (rounded to make things easier). The same features can look very different at different scales and you want your data to look consistent, just like real terrain would.

You then want to decide what scale your eventual fantasy map will be at. As I have said before, you can make any area unrecognizable, but I also enjoy going for more obscure terrain (if you are from the US or Europe of course) to begin with just to make it as fresh and exciting as possible. I got a tile from South America around southeastern Brazil, western India, and part of the Sakha Republic in Siberia (Russia). Does not have to be of a particular place, the goal is to get a lot of stuff to work with and pare it down later. I grabbed some elevation data from GMTED (you can get it here).