A powder room is just a rather more fancy way of referring to a bathroom or toilet room. It’s primarily designed to be used by guests. The term may also refer to a women’s restroom, usually in a building open to the public. Just like in the case of a regular bathroom, the powder room may present different challenges related to its interior design and décor. For example, a narrow powder room is not ideal but not impossible to work with either.
In the case of a narrow powder room, the key is to keep the side walls as free of furniture as possible. It would also be useful if you could avoid using floor furniture and if you could have wall-mounted features instead. Storage could be a problem in such spaces but open shelves are always a nice idea.
If you want to emphasize the shape of the long and the narrow room, then use vertical stripes or other patterns. If, on the other hand, you want to make it seamlessly integrate into the whole interior design of your home, then go with something simple, maybe a combination of colors.