The smaller the pebbles the better! The heat gets less retained and dissipates faster during the nights cool down. And let me tell you: This is a one in a time investment that needs NO maintenance or upkeep at all, just using some bleach on the water once in a while will keep them white and free of mold/green stuff, or best weedkiller... And the best part is that you can do it yourself, with a minor help to place the sacks on the roof...