Glass coasters are 4" square, 1/8" thick, and have rubberized non-slip, non-scratch feet on the bottom. Coasters themselves are made of glass and the feet are made with some sort of sorcery I don't fully understand.
Glass coasters are dishwasher safe in case you like to wash them in the dishwasher, though I'm not sure how the rubberized feet will hold up in the long term to routine dishwashing. Wash in the dishwasher at your own risk.
Wood coasters are about 3.5" square with rounded corners. Wood coasters are not dishwasher safe.