The person you're replying to posted a link which mentions in the "materials and methods" section that the particles in the study were all smaller than 2mm (and

The person you're replying to posted a link which mentions in the "materials and methods" section that the particles in the study were all smaller than 2mm (and 99.6% smaller than 1mm). To compare with your pebble, which wikipedia says is > A pebble is a clast of rock with a particle size of 4-64 mm