Then there’s the titular Empty Room, of which in the story the father was generous enough to provide a figurative layer. It was his idea after all:
“The lung could be seen to be the empty room of the human body, not mere negative space. By filling and emptying with the stuff of the world it stands as the most aspirational organ, in a literal sense.”
Maybe the explanation is telling, but it seemed that Lethem intended to write about the room in figurative terms, subsumed under the father's voice, to address the theories of the close reader: “The empty room, being a tabula rasa, bore aspects of total corruptibility, a potential we’d in childish obedience overlooked until now.” In the story, the empty room is unlike a book-lined attic or a storage room-cum-cupboard: its charm lies in its being an empty, neutral territory; there are no rules, and “there’s nothing you can’t do there." It’s an incubator of sorts, “a vacuum laboratory” that automatically suits to your needs. (At least in that aspect, it bears a semblance to “The Room of Requirement” in the Harry Potter series.)
Despite the father's authoritarian figure, the rules bowed down to his whims. The list of exceptions grew. The rules are bypassed, supplanted: in its formative years, sign-in sheets are posted at the door; it became a buffer zone for the tension between their parents; at one point, it even had a no-clothes-allowed policy—a defining moment on how the story ended, as the empty room became a sort of crucible for the characters to confront maturity/adulthood (or in their father's case, his sanity), only to be cut short by the abrupt ending.
The last sentence spelled out a question that's been under our noses ever since the entire concept of an Empty Room had been formed: a question about its own limits. ("What's it for?" "Can we play here?" "Can we eat in there?")
“Haven’t you ever wondered,” I asked my sister then, "how much stuff we could fit in here, if we tried?”
The room is boundless, without walls and windows; after all, it doesn't just pertain to a literal space as it is a psychological one, a place engineered to cater to their ever-evolving wants and needs. Clearly, they could fit as much stuff in the room, but what stuff? It’s a rhetorical question made pregnant by its lack of answer—that, or the answer is just too obvious.