@Tee Dee G. I like your comment, "guess which ones a current and which ones are inspiration." I am in a similar situation with two very similar bathrooms to choose for. There are three bathrooms total. I was told on this forum that I had too many ideas and should get an interior designer. So here is what I learned from her.
She said it was ok to have 12" x 24" tiles on floor and shower surround in all three bathrooms if I was ok with it. She said the continuity was good for a whole-house plan. She said some people like to have something a little more exciting and luxurious in the master/primary bathroom. I already knew I wanted a smaller tile for the shower floor and she agreed. In all three, the shower floor tile color is very similar to the remaining floor or the same. My husband really loves half-cut pebbles tile so I chose that for all the the flooring for the master bathroom, including the shower floor. We are in the process of finding one that matches the bedroom floor color so there isn't an abrupt change. The bathroom entry is very visible from the bedroom so it's a factor. In that bathroom, the shower tile is not very exciting...subway. Blah... I already don't like it. And it's a sharp contrast in color, which I don't like. I am thinking of telling her we need to tile shop again. I'd rather have less contrast in color.
In the two other bathrooms, things are simpler. All three surfaces in the same color with the pebbles on the shower floor.
I have one bathroom in your shape and feel these are tricky, especially with a tub. My designer told me that if I don't want a tub I don't have to listen to everyone who says I need one. I agree with you that it might be nice to have tile on the front of a tub. I considered matching the tile on the front to that on the floor and shower wall so as to not break up the lines.
The other thing I have strong opinions on in this type of bathroom is that I want an alcove formed by little bump-outs on the side walls to mark off the shower area. Either that or I want the side wall tile to continue out into the bathroom making for a lot of tile. These frameless shower doors are beautiful but they create this issue. If there is no frame, the transition looks unfinished. I was going for a pair of shower curtains and an alcove, until she told me I could ditch the tub.
There is not much room in these bathrooms, so you may opt for no bump-outs, no alcove. Looks like you want a glass door. Look at your inspiration pictures carefully to see how the continuity is created from shower to remaining area. In the caramel room, well, there's just the one color all through. In the white bathroom, there isn't tile on the two side walls but the shower wall and vanity wall are both white, the same bright white, and the photo is overexposed so you can't see the break. I think you would in normal light and it risks looking depressingly small and old-fashioned. The bathroom in this inspiration picture is either bigger or is photographically distorted to look bigger (looking vanity). The third example with green shower tile is really interesting. Here they created continuity on the ceiling! I'm impressed.
I think it is important to relate adjoining rooms to each other and have a whole-house frame of mind and color scheme. You say one bathroom will be black and white with the other one sandy wood tones. That sounds unnecessarily disjointed to me, but maybe you have your reasons. It also seems easy to blend the two by adding a little wood to your black and white one while adding black and/or white to your sandy one. One thing I am learning from this designer is "nothing too much". Black and white is very cold. All sandy is very warm. She is pulling me towards making sure there are warm and cool tones in every room and continuity and flow throughout. She also made sure that if I have two woods in a room like a wood floor and cabinets, I should have them contrast a little: one mid-tone and the other light or dark.

Find a look you like in two tile sizes. Many tiles work together in both large and small formats.

Using a large tile on the shower walls and something similar or the same on the main floor actually makes a small bathroom feel bigger. Abrupt changes in color can be stunning but many people get exhausted looking at tiles when there is more than one bathroom being remodeled. A close friend went through this with her master bathroom and hallway bathroom being done. She used porcelain marble-looking tiles in the shower walls and then a penny tile on the floor. The penny tile bright out the streaks of navy blue. The main floor was a plain white large format tile so it didn’t compete with the shower walls. I can tell you that she and her husband came up with SEVEN different designs before deciding in this one. They all looked good in the tile store, but the “winner” looked best in their bathroom.
The second bathroom was easier because they knew which other tiles they also liked. So that one got chosen more quickly.

Keep in mind that smaller tiles with more grout are better for shower floors. Less slippery. You can “go big” on the other surfaces if you fall in love with something specific.

The first picture you posted is AI so don't use it too literally.

Personally, for your size room, I would do 12 x 24 tile on the floors ( or 24 x 24 max ) unless you want a patterned floor ( 6x6 or 8x8 ). I really like the larger format tile for the shower /tub walls - 48w x 24h - easy maintenance, less grout lines. You can get some great patterns or something that is simple and looks like a marble ( go to the Happy Floors website - they have great pictures) . You can also add some design in the tile for the back of a niche.

I like accent tile walls behind a vanity only if it is in a space on its own.

Black and white bathrooms can be amazing - just come up with the right mix.

Good luck!

You are clever....

1. design recommendations re: tile size? especially are there any "too big" for floors of my bathroom size?

I like the idea of going two different similar colors tiles to create a peaceful space.

2. I need 2-3 tiled surfaces in each bathroom: floor, tub or shower alcove, shower floor and possible the front of the tub. any comments or photos regarding using the same tile twice? - i.e. the main floor and the shower wall? room floor and shower floor? shower floor and wall? there's also the decision of having the same floor in both bathrooms? they are mirror images back to back to back....

I think it can be done nicely and look lovely. I am not a huge fan of black in a bathroom. Black dates quickly while a creamy beige will stand the test of time longer than a black tile.

Here is a bathroom using the same tile in two places

Spring Timeless Tuscan · More Info

Putney House · More Info

23 Downing Street · More Info

That bubbly mass of rocks masquerading as a shower floor in the first photo is a hoot!

Make sure you have a tiler who knows how to lay pebble floors -- they'll have to remove pebbles at the edge of sheets and install manually to make the floor feel continuous.

Here are some bathrooms in your shape. They have bathtubs with nice treatments. I saved pictures with two shower curtains because I liked the look.

Sunnyside · More Info

Tobin Bath Renovation - Olivette, MO · More Info

Mill Valley Kitchen and Master Bedroom Renovation + More · More Info

Druid Hills Master Bath Renovation · More Info

12x24 tiles on a tiny shower made ours look larger. We did penny hex tiles for the shower floor and niche.

I help design and sell tile for a living. Think of spas for relaxation: all the same color tone. If you want a base color, the main and shower floors using the same tile in different sizes work to keep the room looking larger. The shower walls can be the main floor tile and that will allow you to paint the walls and ceiling whatever color you desire and as often as you care to. The photo of the navy wall and ceiling would be a good option for one of the neutrals you have as samples. Artwork can add interest and color too. My other suggestion to clients is to "hide" the niche. Most photos show 2 clear bottles and plants. If you DO change out your products to pretty bottles then by all means show them off. Most people end up having a niche that looks like a grocery store shelf. Another personal preference is NOT doing a mosaic in the niche use the walls tile. I did it in previous home and your cleaning person will have to use a toothbrush to clean soap from all of the grout lines. I am my own cleaning person and hate cleaning grout!

I’d have to ask what the floor material is outside these bathrooms as that visual transition is important. Also, stylistically what is the rest of the house like/the kitchen materials, etc? You need some ‘family’ of materials to create some cohesiveness. Looks like you’d like wood vanities in both? Would they be the same wood/style and hopefully some stain relationship to what’s going on outside these bathrooms?