Mosaic Floor Tiles
While the mortar beneath mosaic floor tile is smoothed out prior to installation, it must first be keyed into the floor by combing it with a trowel. For this, a v-notched trowel in sizes 3/16-inch to 1/4-inch should be used.
The rule of thumb to work by is: the larger the tile, the larger the trowel. A mosaic tile is defined as 2-inches in size or smaller. So a 3/8-inch or 5/8-inch mosaic will use a 3/16-inch trowel, while a 2-inch mosaic would use a 1/4-inch trowel.
Medium Sized Ceramic Floor Tiles
Ceramic tiles 6-inches to 13-inches that are a standard 3/8-inch thick or thinner can be installed using a square notch trowel. The size notches should be 1/4-inch by 3/8-inches.
While the name implies a square notch, it will actually be a rectangle, slightly longer than it is wide.
Large Format Ceramic Floor Tiles
Large format tiles, 16- nches or 18 inches in size or 1/2-inch thick, need a larger square notched trowel. For these, use a 1/2-inch x 1/2-inch square trowel.
Round trowels of this size are also availabke, but are seen less frequently. Both shapes will give adequate keying for large, thick or heavy tiles.