Cidri is a pretty cool idea. It's a non-setting setting; a place so lacking in details that nearly anyone's game can take place there. And as far as shared worlds go it appeals to me more than others because I'm the kind of guy who likes to design his own worlds. I've never much cared for playing in other people's sandboxes, be they published game settings such as Greyhawk, or The Forgotten Realms, or fantasy locales from literature, such as Middle Earth, or Westeros. What I love most about being a game master is creating my own settings, and it's really attractive to be able to do that while still having my campaign set on the same world as everyone else's.
So what would Cidri actually look like as an artificially constructed world that is at least thousands of times bigger than Earth, if not more? This is an important consideration, because the nature of the world will determine it's what the world is like, the length of its days and years, the environment - everything. It's easy to imagine Cidri as just a really, really big planet. But is it? Is that even practical?
Consider the mass and amount of material required to build a solid sphere the size of Cidri. Jupiter, the largest planet in our system, is 1,300 times larger than Earth and has a mass of 1.898 x 10^27 kg. That's a lot of mass, but Jupiter is a jovian planet and is composed mostly of gas and liquid. Cidri is a terrestrial planet composed of siliciclastics and metal, so it will be much heavier. If Jupiter were terrestrial its mass would be about 1,300 hundred times that of Earth, so about 7.763 x 10^27 kg. And since Cidri is said to be thousands of times larger than Earth, it's probably even bigger than Jupiter. Say at least twice as large, and maybe even larger.
That much material would strain the resources of even the fantastically advanced Mnoren architects, and it isn't really necessary or economical for Cidri to be a solid sphere.
A more practical structure for an artificial world would be a Dyson strip, such as Larry Niven described in his 1970 science fiction classic, Ringworld. It is based on a Dyson sphere, which was a thought experiment proposed by physicist, Freeman Dyson; a sphere built around the sun to capture 100% of its energy. Dyson never intended his sphere to be a habitable structure, but the idea of it being so is compelling. Niven made his fictional world a ring because the angular momentum of a sphere's rotation would generate gravity only around the equator, so there's no point in building anything wider than a narrow strip, if people are to inhabit the inner surface.
Let's imagine a Dyson strip with a radius of 1 astronomical unit (the distance from the Earth to the Sun), and a width of 1 million km. A ring this size would have a surface area of 9.4 x 10^14 square kilometers. Earth's surface area is 510 million square kilometers, so our Dyson strip has a surface area that is 1.84 million times greater than Earth's. If we build 500 km high walls around the perimeter of the ring, they will contain the world's atmosphere thanks to the gravity generated by the ring's rotation. If a second ring of linked strips were built with a faster period of rotation we would have hours of light and darkness to simulate day and night.
A sphere around the sun with the same 1 AU radius would have a surface area of 2.81 x 10^17 square kilometers (550 million times more surface area than Earth). In this case though, we'd need some sort of artificial gravity generator since any gravity resulting from the rotation of the sphere would pool at the equator. Also there would be no simple way to create day/night cycles as we can with a ring, so it would perpetually be high noon on this world, and there would be no way to tell time or to navigate.
But what if the inner surface of a Dyson sphere still isn't big enough? What if you want the world to be really roomy. How about we build the sphere around one of the stars in a binary system and construct gravity generators in the middle of the crust so that gravity is exerted on both the inner and out surfaces of the sphere? Now we have a world very similar to Pellucidar of Edgar Rice Burroughs' Hollow World series, but on a much more massive scale. Both the inner and outer surfaces of the world would be habitable, each heated and lit by its own star. The combined surface area of the two surfaces would be 5.62 x 10^17 square kilometers (1.1 billion times the surface area of Earth).
Does an artificial sphere really need to be this ridiculously big? Couldn't the Mnoren simply build a Jupiter-sized sphere that behaves like a normal planet? They could, but there would be some problems with this model. The first is that because there is no solid core, the crust would need gravity generators to prevent everyone from floating off towards the sun, and these need some source of power. The second is that there would be no plate tectonics on such a world, and hence no way to recirculate carbon. In a normal system atmospheric carbon dioxide is dissolved in rain water and precipitated as weak carbonic acid, which eventually makes its way to the oceans. Oceanic carbon is precipitated as calcium carbonate and sequestered as limestone on the sea floor. Eventually tectonic activity subducts one plate underneath another, causing it to melt and release gaseous carbon dioxide back into the atmosphere. Without this cycle the world's atmosphere would eventually become too thin to support life. This is exactly what happened on Mars. Originally Mars had an atmosphere much like Earth's, but because it is a smaller planet and further from the sun, its core cooled off quickly and plate tectonics ceased. The carbon dioxide in Mars' atmosphere was slowly drained away, and is sequestered below ground with no way to reintroduce it to the atmosphere. So in order for life to be sustainable on a sphere or ring world it would need mechanisms to regulate the carbon cycle. These would also require a lot of power, as would the undoubtedly large number of other machines needed to maintain the environment of an artificial world. The beauty of the Dyson sphere or strip is that they can function as Freeman Dyson originally intended: as super efficient collectors of solar energy. The diurnal shade strips of a ring world could have giant solar panels on the sunward side; all that would be needed is a way to transmit the energy to the world's surface. Similarly, a portion of the surface of a Dyson sphere could be dedicated to energy collection, and would received enough to meet all the the energy needs that world would ever require.
So the artificial world of Cidri is unimaginably huge because it needs to be. A sphere or ring built around the sun is the best way to obtain the massive amounts of energy required to sustain the world's life support systems. Whether Cidri is a ring or a sphere isn't really important; either one offers infinite adventure, and far more territory than any adventurers will ever be able to explore. Constructing such a world is a big undertaking, but surely not beyond a people who possess the technological advancements of the entire multiverse.