What you've linked seems to actually directly contradict this; Nomura states "you won't wander endlessly in darkness" in reference to the Realm of Darkness. The source is stating the exact opposite of what your premise is based on.
This isn’t exactly the case. The point of him saying this is because you start at Cinderella’s area. When you simply enter the Realm of Darkness, at some random part, you end up like Riku,
walking through this endless black.
Furthermore, even if this wasn't the case, "wandering endlessly" and variants of it are common phrases - people say that about exploring decidedly non-infinite areas rather often. If this transcript was translated from another language, it's possible these artefacts are a result of translation issues, but in such a case I'd need verification on the validity of the translation and the usage of the terms used. This isn't enough.
Considering the fact that, again, in the same quote, it states that it houses Cinderella’s Castle (again, an entire space-time with independent timeflows), infinite has to be the case here.
Depending on how the word "parallel" is used, it is possible for the Realm of Darkness to be a parallel of the Realm of Light without being the same size or having an equivalent to the Ocean Between; is there more evidence to suggest either of these are the case?
Improbable, really. I don’t see how you could feasibly justify “parallel” not being equal in response to a 5-D space. That would have to make the RoD infinitely smaller, which isn’t ever substantiated. It’s actually the opposite, since, again, Xehanort describes each world as small in response to the infinite Ocean, and how Nomura says how you don’t have to wander in endless darkness because you’re starting in Cinderella’s area.
There’s also this statement, from Nomura:
As for the structure of the worlds, first, the so-to-speak normal worlds—the ones that
Sora, the Disney characters and we live on—are situated in the
Realm of Light. If you picture those worlds as existing on the same level on top of a giant plane, then on a separate level, in other words on the reverse side, exists the
Realm of Darkness.
Again, this runs into issues with literalness and translation. How can you reach the "furthest end" of an infinite space?
Not to pull a whataboutism, but this is common in fiction. Take BlazBlue characters reaching the center of the Boundary, despite it being infinite in size. Or D, from Vampire Hunter D, crossing an infinite space. Or Helios, from God of War, in which his light reached an infinite sized Underworld. Or, even more so, Persona & SMT, where Seth/Kuzuryu and many more threaten to below up the infinite space. By this logic, their attacks shouldn’t have worked, because there would always be more space.
Does "furthest end" here mean actually being at the edge of the Realm of Darkness, or simply being at the point where there is nothing of note left? What context is given about their journey there other than the fact they arrived on foot? Is the "sea of darkness" mentioned just another name for the Realm of Darkness, or is the sea of darkness just a part of it?
It’s called The Dark Margin, the place where the World of Darkness “ends” and links with the Realm Between (that being Light & Dark realms respectively).
Is this an accurate translation, and how do we know? I know that these questions may come off as nitpicky, and I don't like coming off as someone who "always finds something wrong", but there's too much to scrutinise here to ignore - a halfway sceptical approach doesn't lead to thinking this tells me anything by itself.
No issue. It was translated by Gale, in our server. You can ask him if you want.