Below a cross-sectional diagram of an O’Neill cylinder:
The outer circle represents the wall of the cylinder, the star represents the axis of rotation. Because the cylinder is rotating a centrifugal force is generated, which will press people and other stuff against the inner wall. Hence the centrifugal force effectively replaces terrestrial gravity.
When we refer to “land” on this site, we mean the wall of the cylinder.
Not shown in this diagram is that an O’Neill cylinder is divided in six sectors, think of a pizza divided into six slices, three “valleys” – where people are supposes to live – and three windows – through which light enters the settlement.
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What is the practical size limit, and how many people could it hold?
The size-limit depends on the strength of the used material. O’Neill believed, back in the ’70s, that a diameter of 240 km would be possible. However, most serious designs use a radius of about 3,5 km. With this latter size (and length of about 30/35 km), O’Neill believed that a population of 10 million could be possible.