Humanity is an advanced civilization, and has long, long since took to the stars. Their resources are seeming boundless, and their technology is fantastic. For some reason beyond our 21st century understanding, they have decided to perform a very odd experiment. They wish to create a world and strand a few ten thousand brainwashed members of their own race on it, and see how they evolve as a society. The members will have no memory of humanity's past, but will have basic skills (a common language, understanding of simple machines, agriculture, animal husbandry, basic construction, tool making, etc., placing them on par with barbarian Europe during the early medieval ages. They want to watch humanity develop on a planet of the following description:
- Earth like as possible (similar climates, weather patterns, resources, gravity, although it doesn't need earth-like geography, as in the shape, nature, and location of landmasses)
- Physically and structurally stable
- Hidden in its man-made nature enough that it will remain unknown that it was man-made until at least the society reaches early space faring technology (such as what humanity reached in the mid-to-late 20th century)
- And as absolutely large as physically possible
...And they're willing to build this world from scratch.
By no stretch of the imagination does this world need to be a planet in our traditional sense; it's perfectly acceptable for it to be an artificially created structure. The magnetic field this new planet possesses doesn't need to be created by a molten core. In fact there doesn't need to be a core at all! It's even been suggested that making the planet hollow (if such can be engineered, and complications like affected gravity can be dealt with) might allow for a much larger surface area.
In the end the planet just needs to seem like a natural Earth-like planet up until at least the predescribed point in the developing society's future. It's also worth noting that this developing society has no point of comparison as to what a natural planet actually looks like, so as long as it doesn't affect the development of the society they could stumble upon some (hidden) artifacts of the planet being hand-crafted.
Again, humanity has seemingly endless supplies to throw at this project, are in no particular rush to need it completed (a few generations' put towards this effort are perfectly fine), and have extremely advanced technology. However they are still constrained by the laws of physics as we know them.
What would this planet be like to achieve the predescribed goals as closely as possible? What solution might humanity use?