Mosaic Ceramic Tiles
While you will smooth out mosaic tile mortar prior to installation, you still must key it to the correct depth beforehand. For mosaics ranging from 3/8 inch up to 2 inches in size, use a V-notch trowel that measures 3/16 by 5/32 inches. You can install 2-inch tiles using a 1/4-by-3/16-inch V-notch trowel as well.
If necessary, you can use a 1/4-inch square-notch trowel with 2-inch ceramic tiles, but avoid this size for mosaics any smaller.
Ceramic wall tiles commonly come in 4-inch, 3-by-6-inch, 6-inch, 4-by-8-inch and occasionally 8-inch sizes. For the majority of ceramic wall tiles, a 1/4-inch square-notch trowel is adequate. You can also use a 1/4-by-3/16-inch V-notch trowel to install 4-inch tiles, but do not install that size of tile with a trowel any larger.
Install 4-by-8-inch and 8-inch wall tiles with a 1/4-by-3/8-inch or 1/2-inch square-notch trowel. A 1/4-by-3/8-inch square-notch trowel is also adequate for installing 6-inch square wall tiles.
Small Floor Tiles
Ceramic floor tiles are available in 4-inch, 6-inch and 8-inch sizes. Floor tiles are typically thicker than the corresponding wall tiles and need a slightly larger trowel to accommodate the mortar.
Use a minimum of a 1/4-by-3/8-inch square-notch trowel. A 1/2-inch square-notch trowel is also fine for 8-inch floor tiles.
U-notch trowels that are 1/4 by 5/16 inches are adequate for all ceramic floor tile sizes up to 8 inches square. You will require a larger trowel for rectangular-shaped tiles that exceed these sizes.
Large Floor Tiles
Ceramic floor tiles and some wall tiles are also available in 6-by-12-inch, 12-inch, 12-by-24-inch and 16-inch sizes. These thicker tiles require a much deeper mortar to beat the tile into place to prevent the corners from lipping.
Use a 1/4-by-3/8-inch square- or U-notch trowel for floor tiles up to 12 inches in size. For tiles over 12 inches in size, use a 1/2-inch square-notch, 1/4-by-1/2-inch U-notch or 3/4-by-5/8-inch U-notch trowel. The thicker the tile, the larger the trowel required; tiles thicker than 3/8 inch will require larger U-notch trowels.