Mosaic tiles and wall tiles up to about 4 by 4 inches call for a v notch trowel with 3 16 by 1 4 inch notches while larger tiles require a trowel with a square or u shaped notch to obtain proper adhesive coverage.
Trowel size for mosaic wall tile.
If one of the tiles is fairly flat with minimum warping cupping you may meet the proper coverage requirement using a 3 8 square notched trowel.
Finding the right trowel for the job is quite simple when you know that generally speaking the trowel size should match up to the tile size the smaller the tile the smaller the trowel.
Tile trowels come in many shapes and sizes generally starting out from a 3 16 v notch up to the 1 2 square notch.
The larger the tile the larger the trowel.
Tile trowels come in many shapes and sizes generally starting out from a 3 16 v notch 3 16 to 1 4 v notch mosaics tiles up to 4 1 2 glazed wall tile.
If however the other tile has quite a bit of cupping you may need to use a larger 1 2 square notched tile trowel to get proper coverage.