My only experience with rocks is when some friends and I did a ride in Colorado. We did a big loop from Ouray to Gunnison, Crested Butte, Delta, Telluride, and back to Ouray; taking as much dirt roads and trails as possible. Riding through Taylor Park was my first real exposure. At one point, somehow I got a rock under my skid plate and had trouble getting over it. The rock was big enough I couldn’t just roll over it with my rear wheel and it kept rolling every time I tried.
Down here in SE Texas, trails have a lot of above ground roots, but never have larger, loose rocks.