Trowel size for large format tiles

some of my walls are new plasterboard whereas others are the old plaster which is uneven in places.

I’m using 600x300 ceramic tiles.

Which notch size do you recommend?

Thanks in advance

 

6mm - comb butter the back of the tile and comb the wall/surface.

 

should the direction of the comb on the wall and the tile be the same?

 

irrelevant but fwiw try and comb all wall adhesive in the same direction.