Do Warhammer models that have no models have specific dimensions stated any where?
The answer to your question really ends up being no.  Units with no purchasable model have no defined dimensions, and so are up to you and your opponent as to what is reasonable.

However, as of the most recent codex and updates, the Mycetic Spore is no longer a valid unit.  It has since been replaced with the Tyrannocyte, which serves largely the same purposes.  This unit does have a model, and is, as expected, about as large as a drop pod.  If you want to continue using the Mycetic Spore in friendly games, make sure you and your opponent agree on size, line of sight and rules before beginning.

As an aside, the reason for the change between Mycetic Spores and Tyrannocytes are largely legal, and there's quite the backstory behind the removal of the Mycetic spore option.  It's worth looking into if you're curious.