Fantasy and Science Fiction Writing

The answer really hinges on the technology of the humans living on the planet.   With high enough technology, even small asteroids might be habitable.  With stone age technology, the requirements are much stricter.

Low Technology:

• A food source.
• Liquid water, and planetary temperatures sufficient to maintain water in its liquid state – although liquid water may be covered over by ice, or there may be certain times of the year where everything is frozen.  Their food usually requires liquid water in order to grow.  If the humans cannot find food to eat, they will not survive very long.  Above all, everything should not be permanently frozen, nor should it boil.  Humans will be in trouble from heat long before water boils.  Our DNA becomes unstable at high temperatures.
• An atmosphere which: 1) contains sufficient oxygen and doesn’t contain dangerous amounts of any particular substance.
  2) is of sufficient partial pressure of oxygen to push oxygen into the alveoli of the lungs, but is not of sufficient pressure to rupture the alveoli.
  3) gives sufficient protection against UV, solar flares, and cosmic radiation to prevent dangerous alterations in DNA. Although if your society is nocturnal and dwells underground during the day, higher radiation regimes could be survivable even with low technology.
• Food which contains the correct amino acids and other essential nutrients.

High Technology:

• An energy source that the humans can harness in an economically viable manner.
• Matter

With sufficient technology, humans can create an atmosphere that suits them or bioengineer themselves to an atmosphere which contains toxins, or create needed elements using fusion.

But for any of this to happen, the humans must have access to sufficient energy and raw materials.  Without either of these, humanity will fail.  Hint: loss of access to an essential energy source or raw material has potential for a great plot…