I am guessing that any smaller stone will be quickly lost under muck and sediment, perhaps in just a few years. Larger stone can be used to build structures and cavities.
When our pond was made, we set stones on stones to make voids under them for FHM and other critters to thrive. In a few places we made stone houses so I could later drop an underwater camera down and peek in, but it wound up deeper and hard to access. Still good structure for smaller fish.
One thing to think of is stone and structure will get in the way of netting out fish.
