When I'm playing Skyrim and I'm adventuring in Blackreach, I'm always taken back by the sheer size. It's by far the largest indoor area in the game, and is mostly accessible. The thing that threw its potential size into proportion was the Great Lifts. The Great Lifts are parts of different Dwemer cities that lead to Blackreach. There are several Great Lifts that lead to Blackreach, at least four. However, these are quite far from each other on the surface map, and they do not even (come close to) mark the boundaries of Blackreach, as most open to the centre and surrounding area.

So, just how big is Blackreach in lore? If the game's anything to go by, it's easily bigger then Whiterun city. So how big does that make it truly?

http://images.uesp.net/b/b0/SR-map-Blackreach-overlay.jpg

Theres no way to know from Lore info, considering theres little to no info on it in Skyrim.

Is it just me, or does that look like a snake head with its mouth open, with the Silent City (the artificial sun) at the eye?

Blackreach is actually a small world space, not an interior cell.

I definitely see what you mean. Could just be a coincidence, though.