In a small bathroom, less is always more. If you include a vanity, you will probably need to choose a very small unit. Depending on your needs, you can choose the style (cabinet or drawers or a combination) that is most suited to your needs. Recessed medicine cabinets (set into the wall, with an unframed mirror) are ideal in small bathrooms.
Size is important too. It’s important for the bracket to properly support the shelf that it’s holding up. A common mistake is to buy brackets that are either to big or too small for the shelves. If they’re too big then they’ll stick out, and if they’re too small then the shelf could be unstable and dangerous. The projection of the bracket (the part that sticks out from the wall and holds the shelf) should extend to within an inch of the outer edge of the shelf.