How big is Neptune?
Neptune: 49,528 km (162,493,438 ft). Measured as diameter — equatorial diameter.
Source: Wikipedia — Neptune
Where it sits
Among the 16 planets & moons on this page Neptune ranks 5th from the top by size. The largest in the group is the Sun at 1,392,700 km, which is 28.1× the size of Neptune, while the smallest, Ceres, is 939 km.
Stand an adult of 1.7 m beside it and the ratio is 29,134,118×: you would need 29,134,117 people stacked head to foot to match its width — roughly a crowd the size of a small town standing on each other.
In everyday distances
A football pitch is about 105 m long. Neptune is 471,695× that, which puts it at 471,695 pitch lengths from end to end.
Light covers its diameter in 165 milliseconds. A jet cruising at 900 km/h would need 2.3 days to fly the same distance in a straight line.
Next to its neighbours in size
The nearest thing larger than it in this collection is Uranus at 51,118 km (167,709,974 ft), 1.03× the size. Both figures are the same kind of measurement, so the gap is a fair one.
The nearest thing smaller is Earth at 12,742 km (41,804,462 ft), so Neptune is 3.89× as big as it. They sit in the same group.
Against fixed yardsticks
Against three fixed yardsticks — Neptune is 150,085× the size of Eiffel Tower (330 m); Neptune is 1,656,455× the size of blue whale (29.9 m); Neptune is 3.89× the size of Earth (12,742 km).
How this figure was taken
Worlds are listed by diameter. Gas giants are not round, so equatorial diameter is used for them and mean diameter for the rest. All figures are rounded from published mission data.
How the sizes are taken. Each figure says what it measures — height, length, diameter or elevation — and for which individual or convention. Everything is drawn standing upright on one baseline so a single scale holds, including things that are long rather than tall. Values come from the linked Wikipedia article for each item.
Read the rest — common questions
How big is Neptune?
49,528 km (162,493,438 ft), measured as diameter: equatorial diameter.
Where does Neptune rank among planets & moons?
Number 5 of 16 on this site, from the Sun at 1,392,700 km down to Ceres at 939 km.
What is the next size up?
Uranus, at 51,118 km, is 1.03× the size.
What is the next size down?
Earth, at 12,742 km, is 3.89× smaller.
How reliable is the figure?
It is taken from the cited Wikipedia article and states its basis (equatorial diameter). Values are rounded, and where individuals differ the figure is a representative one, so expect small differences from other sources.