What exactly is the difference between a rock and boulder?
Definitions vary slightly, but it appears that the geologic description of a boulder is a detached rock measuring at least 10.1 inches (25.6 cm) diameter.
(Less scientifically, a landscaping source indicates that a rock that you can carry in 2 hands is considered a stone and a rock that you can carry with 2 fingers is a pebble.)
These should be boulders that everyone is comfortable with.