I’m going to respond mostly so your post doesn’t fall too far down to get attention. My knee jerk reaction is that changing the size of the tile below the rail is going to make the wet area look busy. You are already going to have some ”movement” in the Thassos marble tile with the natural color variations. It is described as V3: Moderate variation. I think I’d keep the size consistent.

I respectfully disagree with KSWL. They need to be the same size as shown.

Jumping off bbstx’s thought about the chair rail, how about just pure white rather marble?

You didn’t ask but….not sure I am a fan of floor tile in the niche and I don’t like it there near window.

I prefer niches on a side wall. I like mine right under the shower. It's not so much a design feature as a practical convenience. and on that same note, I like them to blend in with the wall by using the wall tile.

For some reason, I am thinking the larger tile below might be my choice but I would want to see it mocked up before deciding. If doing the 3x6 below, I would probably do tight grout lines and a grout that matched the tile.

Can you mock up with the larger (blue) subway on the bottom and the smaller (white) on the top?

The ’weight’ of having big over small and dark over light would be my concern.

If you're only asking about the size of the white tile, I would go with the larger white tile. The smaller size appears more appropriate for a kitchen backsplash.

The white has to be below and blue above because we have a blue tub that i want the white behind. The shower heads will be on the left and the niche has to stay where it is because we can’t build it in anywhere else as that is an exterior wall with block construction. We are building the wall in to accommodate creating a niche and to straighten it out. i can make it all blue though.

I want to use the thasos tile not just a white because the thasos and blue celeste are the two colors in the floor tile.

Seems opinion is split on size and I really can’t decide.

Appreciating the advice here!!

Would you maybe consider a larger format tile on the bottom, say 12” x 24” or, even better, 24” x 48”? With the smallest and color-matched grout lines, a larger format tile would give the look of a solid slab and provide some relief between the smaller tiles of the floor and upper wall.

I was thinking the same direction as Fun2BHere. And maybe do a mockup of the large format tiles running vertically rather than horizontally for some relief from all the horizontal lines.

I can try to do a mockup of that but we did large format tiles elsewhere and I don't think I want that look here. I do agree with doing the grout very tight though and same color as the tiles for the lower ones.

The bigger the space, the larger the tiles.

I think the 3x6 looks fine in the mock-up and is suitable for the traditional style you are using throughout the house.

I also like larger tiles - the 3x6 tiles look way too busy. The smallest I would consider would be 6x12, but I used 24x24 in my own bathrooms. I think 12x24 looks good, however. The fewer grout lines, the easier it is to clean.

A square tile would be more traditional, and mixing tile shapes will solve the problem. The Blue Celeste comes in 12 x 12. I'd consider using that as the bottom section with small grout lines. It will read almost solid. The tub is a deeper blue so it's okay to use the Celeste as the background. Then you could use a decorative blue tile inside the niche. I'd make the niche longer and shorter if possible.