How can I photograph an empty warehouse interior so its size feels obvious?

To properly relate scale in a photograph, you need either the proper perspective or something of known size that can be compared to the rest of the scene. You have pretty decent perspective, however there is nothing near to the camera...everything is at greater distance. Additionally, there is nothing of well-known size to relate the size of the building to.

If you had a person stand in that expanse of clean floor between the camera and the "clutter" of the background, that would probably do quite nicely to help the viewer relate the sheer scale of the warehouse to something of easily identifiable size. To contrast a human with something else...say the door. Doors come in a variety of sizes and shapes, however common doors are usually less than 7 feet tall. If the door in the left wall is the only thing that viewers can use to truly relate to the size of the warehouse itself, the warehouse will probably seem smaller than it actually is to most viewers. They will relate what they know as the most common size for a door, rather than a 9-foot tall door (which is 35% taller than an average door.)

Relative sizing against an object of well-known size and perspective are your primary tools to demonstrate scale in a photograph. You have perspective...you just need something of well-known size.