The space in any typical bathroom is quite limited. Simply buying the biggest shelves available is not going to cut it. Before you embark on purchasing cabinets and shelves (or get them constructed), it is important to properly measure the space first.
The important thing to keep in mind is clearance. While extra storage is always great, ensure that any cabinets you install leave enough space for unimpeded movement through the bathroom.
Also, pay extra attention to the doors. Each of these cabinets is going to have its own door that swings open, which needs to be accounted for as well. You do not want to end up with cabinets that are so close together (or so deep) that opening the doors intrudes upon the available area.
And lastly, measure the walls themselves. You cannot just fill up every inch of the area available. Mark out no more than sixty percent of the wall for wall-mounted cabinets and shelves; leave the rest free.