Pebbles, Cobbles and Boulders I’ve always loved the Westray beach with “The really big pebbles.” I recently discovered that a pebble can’t be any bigger than 64 mm. I think this one must count as a cobble. Share this:Click to share on Facebook (Opens in new window)Click to share on Twitter (Opens in new window)Click to share on Pinterest (Opens in new window)Click to email a link to a friend (Opens in new window) Related Feel free to leave a Reply :) Cancel reply