Because African fat-tailed geckos are terrestrial, floor space matters far more than height. They spend their lives on the ground, exploring, hunting, and shuttling between hides, so a long, low footprint suits them better than a tall vertical enclosure. A single adult is generally well served by an enclosure offering roughly the footprint of a standard twenty-gallon long aquarium, with more floor area always being welcome.
Hatchlings and juveniles are an exception worth noting. Very young geckos can feel insecure and may struggle to find food in an oversized space, so many keepers start juveniles in a smaller, well-furnished enclosure and upgrade as the animal grows. The priority at every age is that the gecko can easily locate its warm spot, its hides, and its prey.
When planning size, think in terms of a complete temperature gradient rather than a minimum number alone. The enclosure must be long enough to hold a distinct warm end and a genuinely cooler end, with hiding spots at each, so the gecko can self-regulate by moving between zones. An enclosure too short to sustain that gradient is functionally too small regardless of its stated volume.
For most keepers, a single gecko in an appropriately sized, richly furnished ground-level enclosure is the ideal. Bigger is rarely a problem as long as the space is broken up with cover, but a barren large tank can leave a small animal feeling exposed and stressed.