3mm spacers are a nice average if you want the grout lines to be the same on the wall and the floor, I usually use 2mm on walls and 4mm on floors. Using a 10mm trowel is what i would use on the floor but a 6mm trowel for the walls is fine (both square not rounded notch) with a 6mm trowel you will get about 8m2 of tiling done per 20kg bag with a 10mm only 5m2. Maybe due to the size of the tile use the 10mm all over, but not necessary imo.