Regarding the grout, if you're doing a grout color that is in line with the color(s) in the tile it shouldn't look too busy in any size. I saw a stunning photo the other day of a pretty busy granite, light splash tiles and super dark grout so it doesn't all have to be super muted to blend. As for what the lady said, if you love the 6x6 more than the others, do it. I have short splash on both walls of my kitchen due to cabs and a window so 6x6 would never work but I like the looks of them and didn't realize it was a trend, but what do I know? It's not like squares are some fancy new, trendy shape. ;) Maybe someone else will have better advice on that. Here is a link below. Here is a link that might be useful: Mostly 6x6 backsplash tiles

Your link did not capture a single picture, but if you meant this one, it is 3x6. These are 2x4. When you are wanting info on a picture, if you look to the upper right of it, under the title of the pic, "ususally" (but not every time) it will say Questions with a little tab next to it. Click on that and scroll down the page just a bit and you will see where other people asked questions about the pic and the answers. Often the tile is brought up in question on what size, color, where to buy, etc. This can be really helpful when searching for particulars. HTH :) p.s. I agree, you can use any size you want!

Many of the subway tiles come with built in spacers for a small grout line - though you have to do some math to check that you don't have a gap at the top by the upper cabinets and fudge each tile a bit (a very little bit) to make it come out even. Mine have about 1/16 to 1/8. The grout line looks a little bigger after it is grouted. Here is mine: https://www.houzz.com/photos/my-pics-work-in-progress-phvw-vp~57088087 

Kristina, try the bedrosians makoto line. they are matte, a slight variation and 2.5 X 10 https://www.bedrosians.com/en/product/detail/makoto-tile/?itemNo=DECMAKSHW2510M or the Cloe. exactly the size you want. they aren't completely matte, but not glossy either. the color varies a bit, but in the white or gray you shouldn't see a big diff https://www.bedrosians.com/en/product/detail/cloe-tile/?itemNo=DECCLOWHI28G no variation and completely matte, but these are 11" https://www.bedrosians.com/en/product/detail/clara-tile/?itemNo=100000799 https://www.bedrosians.com/en/product/detail/manhattan-mosaic/?itemNo=GLSMANCAS28G if you just want a plain matte white https://www.allmodern.com/wall-decor-mirrors/pdp/hull-soho-2-x-8-porcelain-subway-tile-a000841710.html glazed brick? https://www.tilebar.com/sample-cadenza-salt-cellar-2x9-clay-ceramic-subway-tile-for-wall.html 3x9 matte https://www.tilebar.com/kiln-white-3x9-matte-porcelain-tile.html or they do have a vintage matte clay tile, 2x9 https://www.tilebar.com/cadenza-stroke-vintage-white-matte-2x9-clay-tile.html https://www.tilebar.com/bayou-white-3x12-ceramic-tile.html These are on sale/closeout https://www.tilebar.com/calloway-white-3x10-matte-ceramic-tile.html tilebar has 5 samples for $5. order some of theirs