I'm a little confused. But I can give you my thoughts on the ideas I think you're leaning towards. In a small space it's best to avoid small squares so I'd nix the 4"x4" tile. If you love the pebble stone, I'd suggest only a rectangular section in the center of the shower floor and do a border of the larger scale rectangular tile. I tried to find a sample of what I mean... Notice in this photo large scale floor tiles and a border in the shower. I think it would be nice to do the pebble in your shower with a border of your large scale tile. (Small tiles in a small bathroom makes the bathroom feel even smaller).

From Mid Century to Contemporary Master Bathroom · More Info

I'd say in this case less is more. Because the space is smaller I would stay away from too many elements as it will get too busy. I would keep the same tile on the main part of the floor the same for the shower wall, this will make the space feel more open if you choose to go with the white marble you mentioned. Aside from that you will need some form of mosaic for the shower and the pebble is a fine option, please keep in mind that all natural stone products need to be sealed (yearly at least in wet spaces) so if you don't mind the additional maintenance I would say go for it. If you would prefer to avoid maintenance there are many porcelain and ceramic options that look like marble without needing to be sealed. And make sure to test out what sealer you want on your stone, some are color enhancing while others are not :)

Nice selections. The pebble floor will look great in the shower flooring. I would not use the square with border tiles. For a zen look it is best to keep the material selections very plain and allow the beauty and quality of the materials to shine. I think all of your other choices are great! Best wishes!

The wall tile is a variation of sizes here is a picture of the idea I have. I really want to work in the white bordered marble as it brings a soft look. What wall would this work into if I can eliminate the pebble (maybe)?

Given the size of your room, I would stick with the pebble tile for the shower floor and the tile that the you posted above for the shower walls. This photo does not have the pebble floors, but it has a similar tile for the walls.

Master Bath · More Info