Size of grains

The pebbles on the beach are roughly of equal grain-size because the waves are incredibly good at sorting by grain size when mass density is the same. A storm wave carries and deposits the coarsest grains while good-weather waves only has the power to deposit finer grains. Most of the grains on this pebble beach is probably deposited during storms. You could walk around and see if there are variations along the beach.

Rounding of grains

Pebbles are so well rounded because the waves have moved back and forth so that the grains have been polished and rounded against each other. The softest rock fragments are probably eroded to microfragments. You can have a look and see if you find softer rock grains, as schist or marble, along the beach.