Lay them both landscape.

The size of the tiles is dictated more to the skill of your tiler. Some tilers prefer smaller as its easier for them to get the fall right. Using wastes that are linear like the grates make it easier as its straighter for them to cut the tiles. Having round drains do make it harder to tile a fall in the floor with larger tiles but now days there are so many choices with the waste options for the shower spaces that it allows more choice in sizes of tiles. What size shower are you wanting to go? Will it be more like a 1000 x 1200 rather than the standard 900 x 900??

I am just wondering whether it is a good thing or not to go with solid colours like the dark blue (prussian blue), grey floors and white on other walls all in tiles. I see that a lot of designers seem to do a pattern on one wall, but I haven't found one suitable and whatever I do, I just don't want it to look out of date in 5 years time. I am not sure how about much of the prussian blue tile is appropriate in order for it to be noticeable but not too overpowering.

I like your choices. Prefer the dark tile for the floor with matching grout, easier for cleaning. I do like the combination of the subway tiles in a herringbone layout as pictured. Use the same grout as the floor and also use this pattern in the shower niche. Then just tile the rest in 300x600 white gloss. Simple and timeless and all the colour can come from your accessories.