Island One, Two and Three

I neve bothered with the 'Island' tags, and just referred to them in
increasing order of size:

Stanford Torus: shaped like a bicycle wheel and housing up to 10,000 people.
Actually, to avoid torque effects, there should be two wheels spinning in
opposite directions. Population would be 20,000. A whole wheel could be
used for agriculture.

http://lifesci3.arc.nasa.gov/SpaceSettlement/75SummerStudy/Table_of_Contents1.html

Bernal Sphere: the designs I've seen give it a radius of just under 900
meters and a population of 75,000. But there are also some that are smaller
and house around 10,000 people. Farms are placed in coiled rings along the
long axis outside the habitat.

http://www.l5news.org/bernalspheredetail.htm
http://www.l5news.org/bernalsphere.htm

O'Neil cylinder: shaped like a cold capsule, this habitat type depends on
how big you want to make it. There are some grandiose designs out there,
but why build a habitat to house 10 million people before you even know how
many people are going to want to live in that particular spot? Also, the
O'Neil cylinders should be built in linked pairs rotating in opposite
directions to avoid torque effects.
the first O'Neil cylinders will most likely be more modest sizes than
envisioned by O'Neil.

http://www.l5news.org/oneillcylinder.htm

As for expanding the various settlement types, the torus and cylinder are
the easiest. Just add another wheel to the torus (the axis is very long to
place reactors, factories and radiators away from the habitat), or another
"cold capsule" module onto the end of the cylinder. The Bernal Sphere is
not expandable -- you'd have to build a whole new habitat and attach it to
one end of the first habitat; might as well build a whole new town.