Trowel size come in different sizes to allow you to have a deeper or thiner bed of adhesive on the walls and floors to suit the tile size you are using.
For a standard sized ceramic tile you would normally use a 6mm notched trowel, so the adhesive bed would be 6mm thick to lay your tile into.
For a floor you would normally use a 10mm notch, so the adhesive bed will be 10mm thick to set the floor tile into.
But for mosaic tiles, with them being on a net backing you don't want too much adhesive on the wall because it will squish through the netting too much, so you'd use a 4mm notched trowel so there will be less adhesive to which you would set the mosaic tile into.
Hope this helps.