What does the manufacturer recommend? It’s related to how thick the tile is. And what material are you using to set? Have you ever tiled before?

Usually it says on the mastic / thinset you are using - which depends on the surface you are tiling.

This place is great if you are new to tiling:

http://www.johnbridge.com/vbulletin/forumdisplay.php?f=1

Just a guess.....but probably an 1/8" square notched trowel. Put down a few tiles and take some up to check for coverage. Best is full coverage without any "squeeze out" of morter between the tiles.

thanks Sophie and Avanti. I should also add that these tiles are .45 inches thick.

https://www.homedepot.ca/en/home/p.2-inch-x-4-inch-bianco-polished-brick-mosaic-tile.1000664214.html

these are the tiles. I assume I will need the square notch trowel as the tiles are quite thick

Backbutter. With white thinset only. No mastic. And lay out a gridwork, not just use spacers. Your walls won’t be flat. This isn’t where to start if you have never tiled before.