I’d say it depends on the amount of detailing you want to sculpt onto your maps, whether you’re going for a stylised or cartoonish graphic style where details will be a lot softer, or a more realistic style where the landscape details will be much finer, and the graphic fidelity you’re aiming for… UE4 uses UE4 units for size and scale, which are abstract and not linked to any real world units.
When you create a map it will create a flat landscape mesh with a certain amount of polygons on the x/y axis depending on the size and component sizes you chose. I’d say start with maybe a medium sized map (127 quads, 2x2 sections) and do some sculpting to see if it would have the level of detail you wanted. Considering if you want to go into 10km max zoom on an HD monitor each pixel would relate to about 5 meters at closest zoom. Sometimes when you try to sculpt too much of a gradient difference into the landscape you can get triangulation of the landscape mesh becoming visible, which may or may not be an issues depending on the graphic style you’re going for.
So for flat landscapes you won’t need that high of a poly density, but for hills and mountains, and maybe rivers you might need a much higher landscape poly density, although if you wanted highly detailed terrain features or mountain peaks, you would probably want to use asset meshes that have a higher poly density than the landscape…
There are also some external software packages that you can use, for example World Machine, that you can use to export a height map of your UE4 landscape and they can create a normals map that can add detailed erosion features that you can then reimport into your landscape material…
So I’d say the size of the map you want will depend entirely on the amount of detail you want to add to it, just do some playing to see what gives you the amount of detail you’re looking for…