The minimum, stated plainly
One adult African fat-tailed gecko needs an enclosure of at least 20 gallons in the long format - a footprint of about 30 inches long by 12 inches wide - and that floor area is the true minimum, not the gallon number on the label. African fat-tailed geckos live on the ground. They do not climb in any meaningful way, so a tall tank with the same gallon rating but a small footprint genuinely gives the gecko less usable home than a long, low one. When you shop, compare floor dimensions first and treat any enclosure narrower than that 30-by-12 footprint as too small for an adult, however it is marketed.
Why the floor has to fit three hides and a gradient
The minimum size exists because of what must fit on that floor, spaced apart. An African fat-tailed gecko regulates its temperature by walking between a warm end, kept around the high 80s to low 90s Fahrenheit at the warm hide, and a cool end in the mid-to-high 70s - and that gradient only works if the enclosure is long enough for the two ends to actually differ. The gecko also needs three separate shelters: a warm hide over the heated end, a cool hide at the far end, and a humid hide lined with damp material where it can loosen skin before a shed. Crowd those into a smaller floor and the hides touch, the humid hide warms up, the gradient flattens, and the gecko must choose between being the right temperature and feeling hidden - a choice it should never have to make.
What goes on the floor, and what to skip
Inside the minimum footprint, furnish for walking and hiding rather than climbing. A shallow water dish the gecko can drink from and occasionally soak in, a flat stone or tile under the warm spot, and a piece of bark or a low branch for texture and cover complete the layout without stealing floor space. Keep substrate choices safe for a ground hunter that strikes at insects on the floor - a solid surface or a suitable soil-style substrate kept clean - and avoid piling in tall decor that adds nothing for this species. A secure, well-ventilated lid matters even for a non-climber: it holds humidity in the range this gecko needs, keeps household cats out, and stops feeder insects from touring the house.
When bigger is worth it, and when smaller is excused
If you have the shelf space, step up to an enclosure with roughly a 36-by-18-inch floor. The extra length gives a wider, gentler temperature gradient, room for a second cool-side hide, and space for the gecko to exercise on its nightly walks, and keepers consistently see bolder, more active geckos in the larger footprint. Two situations justify the bare minimum and no bigger: a quarantine or hospital setup for a sick gecko under veterinary guidance, and a short temporary stay during an enclosure repair. Neither is a permanent home. What never justifies smaller is the gecko's size - a fat-tail looks small curled in one hide, but an adult uses every inch of a properly sized floor over the course of a night.
Checking that your enclosure is actually working
Judge the enclosure by the gecko's behavior over a month, not by the tape measure alone. An African fat-tailed gecko with enough space rotates between hides, sheds in clean pieces helped by the humid hide, hunts insects across the floor in the evening, and keeps a plump tail. One that never leaves a single hide, sheds in stuck patches, or goes off its food may be living in a flattened gradient or a floor too cramped to furnish properly - measure the warm and cool ends, confirm all three hides are present and separate, and correct the layout before assuming illness; persistent appetite loss or stuck shed around the toes deserves a reptile veterinarian. Recording the final enclosure dimensions, hide layout, and temperature checks on the gecko's KinsLe profile through Family Sharing means anyone in the household can verify the setup at a glance.