As a good rule of thumb, larger tiles that match the color of the walls. This seemingly eliminates the borders between the wall and the floor, creating a more breathable, seamless space. Also, the larger the tile, the longer the distance the eye travels from one tile to the next. This creates the illusion of a bigger space. Also, a diamond pattern creates the illusion of a bigger room than a checkerboard pattern. In a diamond orientation, the widest section of each tile faces the entrance. This prevents you from measuring the space in terms of tiles across or tiles down. Also, any tiles from Crossville are Green Squared Certified meaning they are eco-friendly and sustainably & ethically sourced.

Would choose rectangle tile pattern rather than a square pattern -- and not necessarily a solid color (multiple shades of gray could work). In this pic, if the longest part of the room is between tub and photographer, the way the tiles are turned (to me at least) makes the space seem even longer.

https://www.pinterest.com/pin/84653667998961757/