Ms Peel Hey! So, resolution of the height map isn't your main concern here. Most artists who make greatly detailed landscapes rarely rely solely on the height map they generate. They use a combo of the height map, as well as textures and objects to add detail. Most game devs who are world building will rely on the height map for most things but add detail at points of interest. This is to reduce the resource requirements needed for their games.
You can throw a 16K map into whatever you want but if you will always be limited by a height map as they don't have the ability to produce details outside of the x, y coordinates. Therefore, to have greatly detailed landscapes, you'll also need to make assets to help sell the illusion. Tried and true textures, rock assets, foliage, atmosphere, things like that.
For reference, most games made, even the really good looking ones, rarely use more than 4K height maps.