Where do you recommend using smaller (1-3”) River Rock and larger (4-6”) River Rock?

This is a personal and subjective question. The following is only our subjective view. Smaller (1-3”) River Rock has a more clean look and is easier to maintain (i.e. leaves). Therefore as an example we recommend and use it around perimeters of buildings under the drip edge. Larger (4-6”) River Rock has a more natural look. As an example, we use it as a practical use for areas with higher intermittent water flows over a geotextile as a Natural Rip Rap. You can mix in larger pieces to mimic a natural dry stream bed.