Charles Dickens-American Notes

"That this state-room had been specially engaged for ‘Charles Dickens, Esquire, and Lady,’ was rendered sufficiently clear even to my scared intellect by a very small manuscript, announcing the fact, which was pinned on a very flat quilt, covering a very thin mattress, spread like a surgical plaster on a most inaccessible shelf. But that this was the state-room concerning which Charles Dickens, Esquire, and Lady, had held daily and nightly conferences for at least four months preceding: that this could by any possibility be that small snug chamber of the imagination, which Charles Dickens, Esquire, with the spirit of prophecy strong upon him, had always foretold would contain at least one little sofa, and which his lady,
with a modest yet most magnificent sense of its limited dimensions, had from
the first opined would not hold more than two enormous portmanteaus in some odd corner out of sight...
Thanks for reading so far!
Q1: Why is the shelf "most inaccessible"? Is is because it is high? I wonder what a state room on a 19th century steam boat is like.
Q2: What does it mean "with a modest yet most magnificent sense of its limited dimensions"?
A million thanks.