Mosaics
Marble tiles are available in sizes beginning at 3/8-inch on mesh-backed sheets. For all marble mosaic tiles measuring up to 2 inches, use a small V-notched trowel. Keep the size of the trowel 3/16-by-5/32-inches when laying the tiles loose. If the tiles are attached to a sheet of mesh, paper or plastic, the size of the trowel is less critical because the grooves will be flattened out by the flat edge of the trowel before placing the sheet in the mortar. In this case, use up to a 1/4-by-3/16-inch V-notch trowel.
4-inch and Subway Marble Tiles
The most frequently sold size of tumbled marble tiles is 4 inches square. Polished and honed marble can also be found in this size, as well as in the popular "subway" sizes of 3-by-6 inches and 2-by-4 inches. For these smaller-sized marble tiles use a V-notched trowel that measures 1/4-by-3/16-inches. Keep the thickness of the tiles in mind; most tiles this size will measure 3/8 inches in thickness. If the tiles are thicker, consider going to a 1/4-by-1/4-inch square notch trowel or a 1/4-by-3/8-inch rectangular notch trowel for better coverage.
6- to 8-Inch Tiles
While not common, it's possible to find marble tiles in 6-and 8-inch squares. More common is the new extended subway tile sizes of 4-by-8 and 4-by-12 inches.
For these mid-sized marble tiles use at least a square 1/4-by-1/4-inch notch trowel. For better coverage, especially with irregularly shaped tumbled marble tiles, use a rectangular notch trowel that measures 1/4-by-3/8 inches. You can also use a U-notch trowel measuring 1/4-by-3/8-inches.
Large Tiles
Twelve-inch polished marble tiles are one of the most common sizes and styles. To get the best mortar coverage with this size, use a square notched trowel 1/4-by-3/8-inches or 1/2-by-1/2 inches. For marble tiles thicker than 3/8-inches or larger than 12 inches square, use a minimum of a 1/2-by-1/2-inch square trowel or a 3/4-by-5/8-inch U-notched trowel. Remember that a thicker bed may be needed for irregularly shaped tiles, so use a larger trowel.