How big is Mercury?
Mercury: 4,879 km (16,008,530 ft). Measured as diameter — mean diameter.
Source: Wikipedia — Mercury (planet)
Where it sits
Among the 16 planets & moons on this page Mercury ranks 11th from the top by size. The largest in the group is the Sun at 1,392,700 km, which is 285× the size of Mercury, while the smallest, Ceres, is 939 km.
Stand an adult of 1.7 m beside it and the ratio is 2,870,235×: you would need 2,870,235 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. Mercury is 46,470× that, which puts it at 46,470 pitch lengths from end to end.
Light covers its diameter in 16 milliseconds. A jet cruising at 900 km/h would need 5.4 hours 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 Titan at 5,149 km (16,894,357 ft), 1.06× the size. Both figures are the same kind of measurement, so the gap is a fair one.
The nearest thing smaller is Io at 3,643 km (11,952,756 ft), so Mercury is 1.34× as big as it. They sit in the same group.
Against fixed yardsticks
Against three fixed yardsticks — Mercury is 14,786× the size of Eiffel Tower (330 m); Mercury is 163,191× the size of blue whale (29.9 m); Earth (12,742 km) is 2.61× the size of Mercury.
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 Mercury?
4,879 km (16,008,530 ft), measured as diameter: mean diameter.
Where does Mercury rank among planets & moons?
Number 11 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?
Titan, at 5,149 km, is 1.06× the size.
What is the next size down?
Io, at 3,643 km, is 1.34× smaller.
How reliable is the figure?
It is taken from the cited Wikipedia article and states its basis (mean diameter). Values are rounded, and where individuals differ the figure is a representative one, so expect small differences from other sources.