I'd make my library closer to 30×50 before giving it five levels most of the time, but the image rocks.
Honestly I'm just not sure how large it can be before cave ins begin, so I chose a safe size.
If I could I would have multiple rooms of 30x50x5, all connected by a central hallway with a small room at the far end designating the main chambers/office of the head librarian, where there's a massive rough stone pedestal, large enough that a dwarf would need a stool to see onto it, with a huge tome holding the locations and references for all the books in the library.
Each major room would have expansive walkways spanning across, with large balconies for finding books on the higher shelves. The rooms are so large they have monstrous pillars supporting the ceiling, with their own staircases enclosed within them that reach to the lower levels where the more powerful darker magic books have been sealed away.
Got a little bored, decided to write a little something for the staircases towards the bottom areas of the library
As you delve deeper down the narrow stone staircase, the musty mildew smell and the thicker more humid air gives way, there is no dust in the air this far down, and it's much more clear. It's had too long to settle. It feels as though the air has been charged with electricity, as though if you touch anything you will be a ground for all the energy around you. Then suddenly you see through the dim light of your torch a glimmer or a sparkle. You rush forwards down the stair to find out what it is, and find that it's an expansive door engraved along the edges with long scripts in dead languages you can't even begin to understand, and encrusted from top to bottom with magnificent gems. There's an image of a massive book on the front with the pages turning revealing light as though it were coming from the book itself. You turn to look back only to realize you haven't been on the staircase for quite a while. When you go back and look up you can no longer see the light at the top. You begin to look around, and can just barely see the top of the cavern. The ceiling is very uneven with stalactites dripping in various places, creating small puddles along the oddly smooth floor. The walls aren't even visible with your meager lighting, who knows how large this place is, this place is old enough even the dwarves who built it are long since dead, assuming they were dwarves to begin with, who knows what else would have ventured this deep but a dumb adventurer seeking magic and treasures beyond his wildest dreams.