Adult Leghorn size: light hens, lean roosters
A fully grown Leghorn hen weighs about 4.5 pounds and a rooster about 6 pounds, which puts the breed at the small end of standard-size chickens. The silhouette is slim and upright rather than round: a narrow body, a longish neck, bright white earlobes, and a big single comb that flops to one side on hens. Even a mature rooster looks athletic next to a dual-purpose breed of the same age, because there is simply less bird under the feathers than the comb and tail suggest.
A laying build, not a meat build
Leghorns come from Mediterranean laying stock, and their size reflects generations of selection for eggs over carcass. A light hen eats less to maintain herself, comes into lay early, and stays active through a long laying life. That efficiency is the trade you are making when you choose the breed: outstanding laying from a small bird, but very little table weight and a temperament that is busy and flighty rather than calm. Judging a Leghorn against meat-breed weights will always make it look underdone, even when it is exactly right.
What a light, flighty adult means for your setup
Adult Leghorns use vertical space in a way heavier chickens never do. A six-pound rooster and his hens can clear a low fence with ease, roost high given the chance, and range a long way from the coop across a day. Plan a covered run or genuinely tall fencing, roost bars set where their flying does them no harm, and nest boxes in a quieter corner, since active hens dislike laying where the flock traffic is constant. Floor space per bird can be modest; escape-proofing cannot.
Growth pace and the point of full size
Leghorn chicks feather and grow fast, and pullets reach close to adult weight at around five months, near the age they typically start laying. Roosters take a little longer to fill out their frame and comb. Because the breed matures early, a bird that is still noticeably small and undeveloped at six months is worth examining for parasites, feed competition, or a simple failure to thrive, rather than being written off as a late bloomer.
Feed and upkeep for a bird that stays this lean
A light adult frame is cheap to run. A Leghorn hen maintains herself and lays on noticeably less feed than a dual-purpose hen of nearly twice her weight, simply because there is less body to fuel before the first egg is paid for. That thrift shows up across a flock and a season, and it is part of why the breed anchored commercial laying for so long. The caution runs the other way: because a Leghorn carries so little spare condition, a hen that misses feed through bullying, a badly placed feeder, or a worm burden shows it quickly. In a mixed flock, watch that the quieter Leghorns actually get their share at the trough rather than being shouldered aside by heavier breeds.
Recording the size of a Leghorn flock
With birds this light, weight is a useful early signal: a hen drifting well under her usual four and a half pounds is telling you something before her laying drops. Weighing a couple of representative birds now and then, and noting weights alongside laying records, turns guesswork into trend. KinsLe's Pet Passport keeps each Leghorn's identity, hatch details, weights, and health notes gathered in one portable packet, ready to hand over at a vet visit or if the flock is ever sold on.