Width is the measurement that matters
Ask how big a pacman frog gets and the useful number is width, not length. An adult female of the common pet species commonly measures 4 to 7 inches across her round body, sitting like a wide, patterned disc half-buried in the substrate, while an adult male finishes at only about 2.5 to 4 inches. The famous mouth spans most of that width, an ambush design for swallowing large prey whole. Weight follows the same split: a mature female is a dense, heavy frog that can approach a pound, where a male feels like a fraction of that in the hand. Any seller describing sex is worth listening to, because with this species it roughly doubles the animal you are planning for.
Fast growth, then a long sit
Pacman frogs do most of their growing early. A froglet the size of a coin, fed steadily on appropriately sized insects and the occasional larger meal, can reach close to adult bulk within its first year, after which growth slows dramatically and the frog's life becomes one of sitting, waiting and digesting. That appetite-driven growth tempts owners into feeding as much as the frog will take, but a pacman that is round because it is obese rather than mature is a poorer animal for it: the eyes begin to look hooded by fat and the body loses its definition. A finished adult should look like a wide, firm dome, not a balloon, and steady measured feeding gets it there more safely than unlimited meals.
Housing an animal that barely moves
Adult size misleads people about this frog's housing in both directions. It does not need a large enclosure, because a pacman frog's natural day is spent buried in damp substrate with only its eyes and patterned back above the surface, waiting for prey to wander past. A modest tank with enough floor area for the frog to turn, burrow and soak, roughly the footprint of a 10 to 20 gallon enclosure for a big female, covers a lifetime. What the bulk does demand is depth of substrate to burrow into, a shallow water dish wide enough for that broad body, and secure screening, because a heavy frog pushing upward is stronger than it looks.
Adopting an adult pacman frog
Adults come up for rehoming once the novelty of a coin-sized froglet has worn off, and they can be excellent, characterful pets for a keeper who understands the routine. The questions that matter are all about food: what prey the frog takes, whether it feeds from tongs or only strikes at moving prey, and how often it has been eating, since a pacman with settled habits can be stubborn about changing them and a frog that has gone off food needs its history understood rather than guessed at. Sex and approximate age settle the size question for good. Through KinsLe, Transferrable Profiles move those feeding preferences and that history to the new keeper with the frog, so its next home starts from what the frog already knows.