Two very different adult sizes
With panther chameleons, the question has two answers. A mature male measures 14 to 21 inches from nose to tail tip and carries that length on a tall, laterally flattened body with a raised helmet-like casque and a rougher ridge along the spine. A mature female finishes at roughly 8 to 14 inches and never develops the male's size or ornamentation. In both sexes the prehensile tail makes up close to half the total measurement, curling into a tight spiral at rest. Anyone choosing this species should decide which animal they actually want, because the male is effectively a different, larger commitment in cage size, food volume and presence.
Locale changes the forecast
Panther chameleons come from distinct regions of Madagascar, and those locales differ in more than colour. Males from some locales are consistently heavier and longer than males from others, so two juveniles of the same age can be headed for visibly different adult sizes. Reputable breeders sell by locale and can tell you the size of the sire, which is the best available predictor of a young male's finished build. This is worth pinning down before the cage is bought: the enclosure that suits a smaller-locale male will be outgrown by a large-locale one, and chameleons do not adapt well to being squeezed into undersized vertical space.
Growth is fast, then suddenly finished
A hatchling panther chameleon is only a few inches long, yet on a proper diet of gut-loaded insects it grows at a pace that surprises new keepers, adding visible size month by month and approaching adult length within roughly the first year to eighteen months. Males keep filling out after the length levels off, gaining the bulk and casque height of a mature animal. That rapid growth is exactly why records matter: a youngster that is falling behind its expected curve, or a female whose weight jumps suddenly as she becomes gravid, shows up first in numbers rather than appearance. Weighing and noting length at regular intervals turns those changes into something you can act on early.
Planning the cage around the adult
Adult size dictates a tall screen enclosure, around 2 feet wide and deep by 4 feet high for a male, filled with layered branches and foliage so the chameleon can climb, bask and hide at different heights. Females manage in somewhat less, but the vertical principle is the same. Because a chameleon will spend its whole life in that planted column, furnishing it for the finished animal from the beginning avoids the stress of repeated moves. Keep the growth notes, feeding changes and any vet findings together as the animal matures, and KinsLe's Pet Passport holds that record in one packet you can hand over at a vet visit or pass on if the chameleon is ever rehomed.