Small enough to surprise you, and the numbers matter more than the word "dwarf".
People hear “dwarf” and picture something the size of a cat. The reality is a compact, sturdy little goat that comes up somewhere between your knee and mid-thigh — small enough to be genuinely manageable, big enough to be a real animal with real opinions.
Height is measured at the withers, the high point over the shoulder, with the goat standing square on level ground. Nigerian Dwarf does sit at roughly 43–57cm and bucks at roughly 43–60cm.
Those figures are the standard rather than an average. Most mature animals sit comfortably in the middle of the range, and an animal finishing near the top of it is still correct — it is going over the maximum that causes problems.
A mature Nigerian Dwarf doe generally lands somewhere in the 25–35kg range, with bucks a little heavier. Individual animals vary with frame, condition and line.
That weight is worth thinking about honestly. It is light enough that one person can pick up a kid and carry it, and light enough that you are not going to be knocked over by an animal that bumps into you. It is not light enough to simply pick up an adult who has decided she is not going anywhere. If you are imagining carrying your goats around, borrow a bag of feed of about the same weight first.
Breed standard. If you intend to show, or to register progeny, height is part of what the registry is assessing. An oversized animal can be excluded, and you will not know until it is fully grown — which is somewhere around two to three years, not at weaning.
Housing and handling. Small does not mean easy. A 30kg goat that has decided to go under a fence will go under a fence. Nigerian Dwarfs fit through gaps larger animals cannot, which means fencing designed for sheep or cattle usually will not hold them. Plan for small mesh at ground level and no gaps a head can fit through — if the head fits, the goat follows.
What you can expect from them. They are a genuinely dairy breed at a small size, and does in milk produce a useful amount for a household. But a smaller animal has smaller reserves, and a Nigerian Dwarf goes downhill faster when something is wrong than a larger goat does. Small animals need earlier intervention, not less.
Kids are tiny — often only a kilo or two at birth, which is startling the first time you see one. They grow quickly through the first six months, then slow. Most of the height is there by around twelve months, but they continue to fill out through the second and into the third year, particularly bucks, who broaden considerably.
This catches people who buy a yearling and think it is finished. It is not, and a well-grown yearling that looks slight will often thicken up substantially.
The two get confused constantly. Both are small. They are built differently and bred for different things.
Nigerian Dwarfs are a dairy breed in miniature — finer boned, longer in the body and leg relative to their size, with a proper dairy udder. Pygmy goats are a meat-type build: shorter legs, heavier bone, blockier through the barrel. A well-bred Nigerian Dwarf should look like a small dairy goat, not a stocky little barrel with legs.
If you are buying on the strength of an advertisement, this distinction is worth knowing, because animals of mixed background get sold as one or the other fairly freely.
Ask for the actual measured height of the parents, and their ages when measured. Scale in photographs is unreliable and everyone knows it — a goat next to a bucket, a goat next to a child, a goat photographed from slightly above all read smaller than they are.
If height matters to you, measure the animal yourself before you take it, or ask the breeder to measure it while you watch. It takes a minute and a tape measure, and a breeder who is confident in their animals will not mind at all.
Then ask the more useful question underneath it: is this animal the size and shape the breed standard describes, or just small? Small is not the point. Correct at a small size is.