I do not think I used anything beyond 2048 and at a slight distance the texture maps look passable enough. With scatter another aspect is blending textures into something more detailed and complex therefore not having the need of using such large texture maps. Of course that will not apply to everything but I think it should help with more organically random and natural textures.
I thought there was a thread somewhere about optimizing image sizes within Photoshop or Photoshop alternative?