Trying to be a bird
There was a comparision map of the four regions on the Sunless Skies subreddit, but I can't find it right off the top of my head. But roughly, the Blue Kingdom is the smallest, Albion is the largest, and the Reach/Eleutheria are about the same size.I find I kind of prefer the Reach to Albion. In the Reach it felt like I was always heading somewhere and something was always happening, while Albion is like 90% wandering through empty void without anything close enough for your scout to point out. Maybe Eleutheria or the Blue Kingdom will go back to being more claustrophobic.
In terms of what fills them. Eleuth has somewhat more empty space than the Reach, and the Blue Kingdom is about as equally mazelike (but packed into a smaller space).