Instability of long cylinders

> What are the enabling factors and technologies for construction of a
> space megastation?
>
> Megaengineering is more than just a simple scaling up off smaller
> projects. Just for cost, there is a point where more is even more
> expensive.
I may not be understanding the question adequately, but if you're asking is there a size where it's not particularly economical to just keep going bigger, O'Neill felt that point might be reached at the Island Three scale. One of the things which made him feel justified in projectinggoing up to thatscale even within the constraints of roughly contemporary technology is that past a point you quit having to engineer a radiation shield as a separate structure (and separate line item on the budget) because the main structure and the depth of atmosphere become sufficient to provide enough protection from radiation.