Measure the door. Make sure that you get both the width and height of the doorway. If an item cannot fit in the doorway, it will become an extremely frustrating situation.
Measure the width and depth of the room. Take into account any small changes in the layout throughout the room. This will impact your ability to store items in the room.
Measure the height of the room. Although it seems natural to concern yourself with the width and depth, you can also store items vertically. Make sure that you account for any light fixtures that may be in your way.