well, they have to be at least a pixel in wilbur or it can't register as a terrain feature.

so.... to be "realistic" take the smallest feature you want to depict and call it 1 pixel, then work up from there.

But really, maps are maps, not photos. They are meant to represent. Many, many rivers can't be seen from altitude equal to the map shown. The river valley/erosion usually can be, but often not the water itself.

-Rob A>