Middle drum length: 4km - 60km
Middle drum diameter: 1,5km - 12km

Color Coding:
Orange = Docking facilities
Red = Industrial and power generation facilities
Blue = Liquids and gaseous ressource storage
Yellow = Internal habitat cylinder lighting system ( Outside of inner cylinder serves as lighting for the outer habitat cylinder )
Green = Inner and outer habitat cylinder
Clear transparent outer cylinder = Pykrete / asteroid rock / steel outer armored shell

Generalized Design and Layout:
The outermost shell of a stacked O''Neill habitat is a stationary drum. Often it's not even a separate object, but just the surrounding walls of an asteroid, or a mesh net with stone rubble stuck together with ice. It's inner diameter is usually a few hundred meters wider than the middle drum. Heavy industry is located in the carrying structure of the outer shell, where also storage tanks and docks are located. The carrying structure also contains most of the hydroponics for food production.

The middle drum is something with a diameter between 1.5 kilometers for a small, and up to 12km for a large stacked Habitat. This area, due to its giant surface, usually contains natural areas, such as forests, plains, oceans. It's nature reserve, luxury goods farming and holiday resorts.
It's roof is formed by the outside of the inner drum. With older habitats this "Sky" is simply blue painted and has belts from where indirect lights illuminate the ceiling, resulting in a daybright sky. The sun is mimiced by arrays of lamps, which switch on an off to fake the movement of the sun over the sky. Light curvature allows the fake sun to be visible from along the whole length of the drum. There are typically three fake suns, and they move with a slight spiral motion over sky. There are only few spots where somebody on the ground would be able to see two sun images.
More modern habitats use hilarious numbers of LEDs to produce a selfilluminated blue sky as wel las the yellowwhite fake suns.
However, they are also able to produce dusk and dawn effects, which the older habitat designs cannot produce. Both old and new Habitat designs are able to mimic night skies.

With the "sky" being at least 500m above the ground even in small habitats, and over 2km overhead by the largest ones, details on the fast counterrotating inner cylinders outside are barely visible.
To generate winds and even storms as well as to distribute rains, so called "Wind Vanes" can be lowered which move the air and serve as sprinklers. Cooled down, they produce snow for the winter seasons.

The inner drum has a single rod in their center, which carries various things such as sprinklers, wind vanes, but, most of all, the lamp arrays that generate the general days brightness as well as the fake sun.
However, in the inner drum you can see much further around the center, so its population is much more aware of the cylindrical structure of their world.
The innermost drum contains most living spaces, light and service industries as well as, designed as recreational areas, basic food production in the form of fruit bearing trees and bushes.

Construction materials:
Primarily steel and aluminium.
Larger diameter stations use maraging steel, glass fibers, basaltic fibers or selected polymers for the drums.
Purely carbon based materials such as graphene or carbon nanotubes are avoided, as most ACs deem it too easily possible for smaller lifeforms such as bacteria to mutate enough to digest carbon fiber materials.

On earth, dual cylinder O'Neill type stations were originally designed to be solar powered, using large windows in their walls to reflect light in.
The tigers civilization did have the beginnings of workable fusion reactors when they started to design the template for their equivalent to earths O'Neill stations.
They also had excellent LED and EL type light sources, so they never contemplated to design their space stations with large windows aside of a few experimental designs in favour of solid metal walls and internal lighting.

Thus they were able to consider an alternative design to what on earth is called an O'Neill station:
The stacked drum habitat.
The innermost, thinnest drum acts as the "sky" for the middle drum. It is rotating in the opposite direction than the middle drum. Also, it is rotating at a faster rate, so that the spin gravity in its smaller diameter drum is the same as in the middle drum.

The outermost drum is not rotating and not pressurized. This outer shell is usually low grade metal, molten rock and pykrete, fiber reinforced ice. Leaking athmosphere from the inner cylinders can be reclaimed and pumped back.
In case of extreme radiation storms, there are emergency hydrogen reserves kept at the ready to flood the outer shell for additional protection within minutes, to be reclaimed after the danger passed.

The stacked drum setup is contra-rotating to cancel out, or actively control, gyroscopic or vibratory effects, such as precession in an orbiting setup.

Simplified size classes
1,5km diameter middle drum, 500m inner drum, 4km length : Village / Sanctuary
2km diameter middle drum, 1km inner drum, 8km length: Town / Sanctuary
4km diameter middle drum, 2km inner drum, 16km length: City / Biome
8km diameter middle drum, 4km inner drum, 30km length: Island
12km diameter middle drum, 8km inner drum, 60km length: Archipelago


There are many habitats that are shorter whilst having a larger diameter.
The length is more relevant to its classification than its diameter.
But the diameter is most relevant to the considered comfort, as larger diameters mean that you can see less of the opposite side of the drum.
An island class habitat with 30km length and 8km diameter is so large that it can afford a complex lighting system and artificial magnetic field on its inside as to allow bees navigating after the sun and pidgeons to navigate after the magnetic field.
Its innermost drum would still be a colossal 4km diameter, meaning a 2km average height between the middle drums floor and its ceiling. And at 2km "sky height" only few birds would really encounter problems.