The size of cage that your ferret or ferrets need depends on how much time they will spend in it. Ferrets are intelligent and inquisitive. They will not thrive in an environment that is devoid of stimulation. A lone ferret in a large cage with no friends or owner interaction will become listless and may even get sick.
If your ferret's cage must be small, consider a ferret playpen where he can safely spend time in a larger environment without getting into trouble. If you can afford a larger cage, give him as much room as possible with as many hanging hammocks, toys, and tubes as possible.
Ferrets kept in cages indoors can suffer from vitamin D deficiency and get bone deficiencies. Sunlight is important to his mental health and his physical well-being. Take him with you outdoors if possible or wheel his cage into the patio or deck (taking care not to leave him in direct sunlight where he can get overheated).