What size terrarium do I need for a bearded dragon?

Across r/BeardedDragons threads we read, the community consensus is that 4x2x2 (roughly 120 gallons) is the bare minimum for an adult beardie, with larger enclosures like 6x2x2 (180 gallons) preferred when space and budget allow. A 40-gallon breeder is only suitable for juveniles.