(This information is from Ferrets for Dummies by Kim Schilling, Chapter 6; "Home Sweet Home: Setting Up Your Ferret's House".)
Ferrets are carnivores, so they should be housed with other ferrets or by themselves (no mixing them with other small animals). Even if they won't feel the need to hunt another animal, they could play a small animal to death.
Cage's MINIMUM size is 3 ft wide, 2ft deep by 2 ft high for 1 ferret, and more space is MUCH BETTER.
It should have good ventilation. You should avoid small openings between wires (heads can get stuck) and secure doors with snap bolts, the snaps at the end of dog leashes. The cage shouldn't be made from wood because of the impossibility of achieving proper sanitation with one [of the aforesaid wooden cage] and the possible harm to the ferret if the wood (which in addition may contain harmful chemicals) is chewed and swallowed.
Avoid using metals with lead.
The best cages [says the author] are those made of sturdy galvanized wire. If the floor is made of wire the squares should be no bigger than 1/4 inch by 1/4 inch. The wire can be sensitive on their pads.
Multilevel cages with multiple openings are best.
Ferrets should be indoors. They shouldn't be exposed to direct sunlight with no relief. This continuous sunlight exposure can cause heat strokes or heat exhaustion. If the cage is exposed to direct sunlight during part of the day, make sure the rest of the cage (more than 2/3) is free from direct exposure. You should also avoid drafts and moisture.
Cage Stress- "Cage stress is often associated with the ferret's inability to escape to a safe place. Some signs may include constant pacing back and forth and gnawing on the cage bars or scratching incessantly at a corner of the cage. It may even have sores on his head and face from trying to push its way out of its confines." Bottom line - provide hideys: fluffy bedding piles, snooze sacks or covered hammocks. You can use wooden bird nest boxes but remember that ingested wood is harmful, so at thie first sign of wear you should instantly remove them and not put it back ever again.
Remember to place the cage in an area with much human activity so the ferret won't feel left out.