You can check flatness by taking a pair of tiles and placing them face to face or back to back. If there is any gapping around the edges or in the center when you look at them from the side, then they are not perfectly flat. That doesn't mean that they can't be used. It just means that they will look better if they are not laid in an overlapping pattern where you get lippage, and it is important, if they are bowed, that it is consistent from tile to tile. I did a small bathroom in 18" square tiles that were perfectly flat and I still chose to align the tiles in a standard, straight set pattern. Lippage

A tile 3/8" 18" wide and 36" Long weighs about 20 pounds. Two people could stand shoulder to shoulder in that space. I would think two people would average out to about 350 pounds. What happens when you fill a glass of water and place it in the bathroom on the floor. Can you walk around the room without spilling any water? Water should be to the top. Can you dance around the room?

Thank you for the input. I am leaving it. Contractor seems to think veins should be random. The shower was done first a few years ago, we are doing the rest of the bathroom now and actually the large square 24x24 tile is exactly the same porcelain tile as the small herringbone so they coordinate in color. I think a large tile on shower floor would be slippery? I like the grip that all the grout offers.

I'm looking at one of these for wall tiles: https://www.flooranddecor.com/porcelain-tile/biscayne-blue-porcelain-tile-100996982.html?gclid=Cj0KCQjwqs6lBhCxARIsAG8YcDiwdrgYQo2mecCMiy3ESKEcKC7fsuqCabYM_N85EyiVYi0PiaeKnjEaAv1wEALw_wcB&gclsrc=aw.ds https://www.homedepot.com/p/MSI-Coastal-Blue-12-in-x-24-in-Glossy-Ceramic-Floor-and-Wall-Tile-16-sq-ft-Case-NHDCOABLU1224C/320916459 Looking at this for floor tile https://www.flooranddecor.com/porcelain-tile/opal-gray-porcelain-tile-100888635.html Looking at something like this for the accent panels https://www.flooranddecor.com/glass-tile/chateau-blue-2-in.-hexagon-recycled-glass-mosaic-100550714.html https://www.flooranddecor.com/glass-decoratives/navy-pier-glass-mosaic-100463215.html I really think a 4th tile for the shower floor is just too many textures, shapes, and colors, but not sure if I can use the flooor tile or accent tile on the shower floor.