The short answer on least killifish size
A least killifish stops growing at a size many keepers find hard to believe, with adult females reaching about one and a quarter inches and adult males nearer three quarters of an inch. Females look noticeably deeper in the body as well as longer, especially when they are carrying developing young, while males stay slim and streamlined. Because Heterandria formosa is a livebearer, a tank can hold several generations at once, and the fry are smaller still, which makes the whole group look delicate beside standard community fish.
Why females outgrow males in this species
Female least killifish grow larger because their bodies need room for internal development of fry, and that single fact explains most of the size gap you will see in a home tank. Males invest in agility rather than bulk, darting among stems to court females, so a mixed group shows a clear two-size pattern once everyone matures. Young fish reach close to adult length within their first months when food is steady, then growth slows sharply, so a fish that still looks like a juvenile after that early burst is almost certainly a small male rather than a stunted female.
What that tiny adult size means for your tank
A least killifish group needs far less swimming room than larger livebearers, yet the tank still has to protect fish this small from filters, currents, and hungry neighbours. A gentle sponge filter suits least killifish better than a strong intake that can trap fry, and dense floating plants give the smallest fish cover at the surface where they like to pick at food. Choose tankmates from other calm, small-mouthed species, or keep a species-only setup, because a fish barely an inch long can be chased or eaten by community fish that would ignore a guppy.
Feeding a fish that stays this small
Least killifish need food sized to mouths that cannot handle standard flakes whole, so crush flake finely or offer tiny live and frozen foods they can pick at through the day. In a planted tank, least killifish also graze on microscopic life among roots and leaves, which suits their natural habit of frequent small meals rather than one large feeding. Watch females after meals, since a rounded belly that keeps growing over weeks usually means fry are developing, not overfeeding, while a male that stays pinched and hides should be checked for bullying or poor water.
Planning ahead for a fish measured in fractions
Choosing least killifish means planning around fragility as much as space, because nets, siphons, and tank moves all carry more risk for a fish this light. Use a soft fine mesh or a cup when you must move one, keep a tight lid since startled least killifish can jump, and photograph the group when you set it up so changes in numbers are easier to spot among plants. Should your circumstances change and the group need a new home, KinsLe Transferrable Profiles helps pass each least killifish record onward so its next owner starts with the full story.