I think that size laid horizontally across end wall would work. It all depends on layout. I prefer to have some lasted walls on bathrooms

How often would both doors be open and someone standing at the place to see both walls holding a set square to judge the tiling? If you want the large tiles as a feature, then have them.

Have you looked at 200mm x 400mm tiles. This would have less grout lines, but be a different aesthetic.

@dreamer the large feature isn't my concern. I'm wondering what sized plain white tiles would suit best.

As suggested previously look at 200mm x 400mm

Match your grout lines up the wall opp door if tiles go to floor there. But can u post a plan for better comments.
Grout lines are tricky as need to consider where tiles are cut depending on room size and layout To avoid issue go with large rectified tiles with minimal grout on floor