Well, the Void is "the state of Not-being, outside Creation or Ea" (History of Middle Earth Volume X: Morgoth's Ring; Myths Transformed). Therefore it can't have any size, and it's not meaningful to compare its size to the Timeless Halls or to anything else.
The Timeless Halls themselves, being outside Ea, presumably have no physical form that could be defined in terms of size. At the very least, we have no "physical" descriptions of existence there.
(The references to Abyss and Firmament in the Ainulindale are figures of speech; it's far from clear that these are actual sub-regions within the Timeless Halls.)
The whole matter of existence outside Ea is never made clear. Tolkien says that even the Elves who wrote down the Silmarillion weren't entirely clear on the matter: from the Myths Transformed essays on Morgoth:
We read that he [Morgoth] was then thrust out into the Void. That should mean that he was put outside Time and Space, outside Ea altogether; but if that were so this would imply a direct intervention of Eru (with or without supplication of the Valar). It may however refer inaccurately * to the extrusion or flight of his spirit from Arda.
- Since the minds of Men (and even of the Elves) were inclined to confuse the 'Void', as a conception of the state of Not-being, outside Creation or Ea, with the conception of vast spaces within Ea, especially those conceived to lie all about the 'Kingdom of Arda' (which we should probably call the Solar System).)