You probably won't want the niche in the exterior wall due to insulation. And you probably already know that you've got a HUGE shower, so expenses are going to add up very quickly. Is 3-7.5 really the door opening? Or is that the width of the shower? Either way, you've got upwards of 26 sf of shower, and a "standard" shower is more like 12-16 sf. (I say this having recently redone an existing shower that's about 25 sf. Along the way, I started thinking "how is this adding up so fast, and why is it taking so long? Duh! So if you're concerned about cost, you'll want to address the size first.)

I love the marble hex, but I probably wouldn't put it in the shower, due to the issues that Nancy mentions, among others. And I love marble, warts and all. I just don't want it in my shower.

For tile choice, keep it simple, because it's open to the room. I did a 3x1s2 subway-ish with a wavy surface and I think the scale is just about right. I did gloss white, and if I had it to do over again, I might have opted for one of the other pale neutrals in that line. It's quite budget-friendly for a big expanse, and everyone seems to like it:

http://www.anatoliatile.com/collections/marlow/

**it does not come in a bullnose, so consider that! There is a pencil liner, however. You've got a window to trim, so that will require some planning.**

I'm not convinced the "large format" tiles that are so popular would suit your desire to keep it not-too-modern, plus they sound like a PITA to install. Do one niche in the pony wall where you won't see it from outside the bathroom. Niches are to showers what dormers are to roofs: expensive and time-consuming little buggers, so make sure you really want it, and that it's exquisitely detailed. I love the big window in your shower. I have something similar and it makes for such a nice way to start the day.

Get with a good local tile shop and make sure your installers are certified with the tile equivalent of a PhD in everything from the waterproofing to the itty bitty corners of your niches, benches, and drains. Set yourself up for success and you'll have a gorgeous bathroom in no time! Just kidding, it will feel like it takes forever--so make sure you only have to do it once ;-)

Good luck!

I used a 3 x 12 (or maybe 4 x 12?) tile with a tiny bit of ripple in it on my room walls. The color was Pearl in the Piccadilly series from Olympia tile. I used it with Carrara and it looks great. Here it is with the Carrara chair rail and baseboard. The wall plates for outlets are white ceramic, so you can see how the tile is just a light gray. In your shower using a 12 inch tile, I would do a 1/3 running bond, not the 1/2 running bond we did on our walls. We do have a tiny bit of lippage. Our grout was DeLorian gray, a light-to-medium gray color.

I believe I paid about $4.50 a square foot. So it is about four times the cost of the least expensive subways.

Going with a light gray wall tile, you could do the floor in a white hex, like this unglazed porcelain from Olympia. Or if you stick with white subways, the dark gray here could also work. You would have to hold the porcelain hex up to your marble floor tile to see if they go well together. (Since I had their site open, I thought I would see what they had in porcelain hex.) Olympia hex unglazed tile, three colors.

Hmm. You'll never regret the big shower--no judgement here! And you'll enjoy that Bluestar too ;-)

Re: the tile choice: is it going to be curbless? And do I understand correctly that the marble hex would be in the non-shower area? In a space like that, if I were to use two materials close to one another, I'd want them to be different. Does that make sense? Maybe a 4" marble hex in the non shower, then a smaller porcelain hex in a color that contrasts inside the shower. I feel like it's a big room for the little hex, not to mention it's rather a labor-intensive mosaic.

And it's worth noting that if you did decide to do the same marble hex throughout, it's quite likely that the floor in the shower is going to look darker or splotchy, which may not please you. That's one reason I'm asking about the curb--with no visual divider, it's going to be really obvious. And I don't think it'll be too busy. In fact, having a different material in the shower may relieve the monotony of a relatively large space. What about a porcelain hex that resembles bardiglio inside the shower? A medium grey, that is.

Let some pros weigh in on the material choice, and ask your installer. I think some will require you to sign something that says you've been made aware of the pitfalls of natural stone in a shower. If you're planning to be there forever, make sure you're going to love it!

(Full disclosure: I recently ripped out an entire bathroom that was done in slate. Toward the end, the state of it and then seeing the demolition made me a firm opponent of natural stone on a shower floor. And I've since been educating myself through the many bad experiences I see here. It's really hard to clean and maintain.)

Sounds like a fair deal. Heated floors are awesome!

And glad you've had conversations with tile guy. Sounds like you're in good hands.

OK then the easiest thing I'd recommend would be to find something else for the shower floor itself and just eat the difference, if it's a pain to return the marble. Pick another floor for the shower, but not one that tries to match the marble hex. Things want to be either exactly the same, or different enough that it looks intentional.

Glad you like the wavy subway! I think it looks great with marble, which is what I used to trim out (jambs, threshold and niche.) Happy to share pics if you're curious ;-) The only reason I mention the non-white colors is that I found it almost blinding at first, with the high window and the lights not yet on dimmers. I was used to the slate sucking all the light out of the room. There's also a matte version of the white (Cloud Matte,) which is quite elegant with marble.

At the risk of being a Debbie Downer, don't do the marble inside the shower, unless your tile guy feels super confident about it. I don't want to see your name come up on this list of unhappy people:

https://www.houzz.com/discussions/query/marble-shower-floor

Like wavy tile too and its pretty affordable. A bench and accent strip may look pretty too. If you dont go with marble, you could do a pattern like in first pic. I suggest going with what you love though. People warned me about my dolomite countertop...is it harder to take care of yes, is the gorgeous beauty worth it - absolutely. But Im no pro and dont have experience with marble floor tiles either.

bench/floor option

bench

accent idea

accent idea if wavy tile is too much for the entire space

https://www.houzz.com/discussions/bathroom-remodel-ready-to-reveal-laundry-too-dsvw-vd~5591224

There are a bunch of photos here. Scroll down for more. My shower is about 5.5' x 4.5' so about the same size, just not as long and skinny as yours. I'll look for a picture of the niche. I don't think I included one.

Good luck out there! I've seen some porcelain that really knocks my socks off. It seems to get better every year.

Take a peek at the shower I posted on my projects. That might give you an idea.

I tested marble. No way could I live with it. Even with maintenance it will still etch and look ugly.

Post more pictures of the shower construction. It looks to have multiple issues. That’s more important than what you put on top.