Small bathrooms are hard to design. On the 1 hand, since they’re compact, you spend less on materials as you’re using less stuff. On the other hand, small bathrooms are small, and nobody enjoys being cramped.
Avoid having anything which sticks out in the walls particularly in rather narrow bathrooms.If that you are in possession of a small space using a tall ceiling, play up that by painting the ceiling a light blue with some small clouds to draw up attention.
To provide the illusion of more space eliminate as much visual clutter as possible when selecting fittings and stuff: Clear shower stalls with minimal components; swinging doors instead of jump when at all possible.