Short answer: WM is not made to create such detailed textures, so this won’t work, no matter how much you try.
Long answer: WM is a “macro scale” tool, meaning it’s to be used to create big features in your terrain and the smallest feasible features you can desire to get out of WM, are between 1 - 10 meters, depending on the size of your terrain. Just creating terrain in WM is not enough, you must texture it in some other tool, for example, a game engine. In there, you can place/paint high quality textures on the terrain.
So what can you do with WM in regards to texturing? You can create masks, or better said, guides, for your in game textures. Meaning you create a black and white image that will guide you were to put certain textures (or use it to automatically place textures). This is the point where resolution becomes important, as you want your masks/guides to be of an acceptable resolution. Having a 100×100 m guide isn’t helpful if you want to use the guide to help you texture flow lines that are 5 m in width, and the same goes for river banks and such. So instead of trying to create a diffuse texture in WM, use WM to create general guides for the texturing you’ll be doing inside the game engine.