For a small space I would recommend a wet room. I wouldn't have a bath as that will take up too much space, just waterproof the whole room so it can be used as a shower room with basin/toilet too (remember to have a drain)
Given the restricted space, bare nececities should be essential. To best save on space, the toilet should be reasonably adjacent to the toilet. Think of an airplane bathroom. Shelving will give you room for towels, although keep them high, so that it doesn't intrude into the open space.
As the space is not big, don't lay big size tiles on the floor because the room will look like very small. Better not provide any design tiles. You may lay offwhite colour 1 footX1 foot tiles and little dark shade tiles on the bathroom walls.