Where are you shopping? There should be a large selection at a better tile store. The slate look is something that has always been around and the larger format porcelain tiles (work with porcelain...they have fewer issues with expansion/tenting). These are naturally rectangular. If you want a tri/multicoloured slate-look porcelain, you will need to purchase multiple colours in the same size to get this look. Again, a higher end tile retailer (look to a kitchen/bath store if you don't have other options) and take this photo with you. A professional tile sales person should be able to come up with several options to get you the look you want....but be prepared to buy three of four different colours (and be prepared to shuffle them for the installer) to get this look. The big box stores like Lowes or HD will be a difficult place to source this type of slate look.

There is some helpful information about Sintered Stone vs Porcelain here: https://www.tenax4you.com/compact-surfaces-sintered-stone-porcelain-compared-s/2092.htm That website has several articles making comparisons between different materials. Might be a page to bookmark and share with others since the properties of natural stone, engineered stone, and other man made materials can get pretty confusing sometimes. Hope the information helps future readers.

I like those last 4 x 4s on the floor, Beth. But they are not the pure white glazed tiles, they have some variation in color and shading. And they are not paired with identical wall tiles. I don't think that using the same 4 x 4s on both will work well. It will look like the homeowner found a sale and either bought too many or could not find the funds or the imagination to do a different floor. Organic Cook, how about using a large format hex tile on the floor, like the ones in the green and black bath and the white baths just above? It has a nod toward the vintage, by going hex on the floor, with a foot in the modern by not doing mosaic tile. They come in 8 inch size (maybe larger), so will decrease the number of grout lines in comparison with the vintage mosaics. You could look at a nice tile with some variegation, like the 4 x 4 floor above has. You could look for a color that is related to whatever the floor color is in the hall, so that the floor color flows between the rooms. If the hall has a wood floor, look for a tile that has about the same amount of color as the two tiles in the last bathroom in Beth's post above mine. Not a wood-look tile, but a glazed or color-body tile that has tones that tie into the hall flooring. Another idea would be to go back to your original choice of gray and white. Here are two 10" hex tiles from Tile Bar that would look good with white wall tiles.This one is a cement-look porcelain at $6. I love the variegation in color. Link This one looks like it may be beige, which would go with some wood floors. This one is at Home Depot and is more expensive. $15.95/sq ft. If you go to this link, there is a video that shows it grouted with both white and dark charcoal or black. Wayfair has this Carrara-look hex on sale. They show it with dark grout, too. $7.51 sq ft. I can't get an image saved to my computer, so I can't post it.

Many blues..couldn't find 2 by 8 though..they do have 4 by 8.. https://www.daltile.com/product/Festiva?shape=Rectangle&size=4-1/4%20X%208-1/2&color=Sea-Mist another collection of theirs..only two "true" blues, but cominfg in 2 by 8 https://www.daltile.com/product/Color-Wheel-Collection-Linear that's sadly too long..otherwise I love the color, and can come lower than listed if you can give a name of a pro working with you https://www.annsacks.com/products/tile-stone-mosaic/context-field-tile#/context-rectangle-field%2Fgloss-jasper 2 by 8..but only one is somewhat-light-icy-blue.. https://www.annsacks.com/products/tile-stone-mosaic/davenport-field-tile now these are ribbed which I've no idea whether you'd want..only two of them, again, are very subdued blues-one is very light, one is pretty dark. The price though is doable https://www.annsacks.com/products/tile-stone-mosaic/savoy-field-tile