How big is Mars?
Mars: 6,779 km (22,240,814 ft). Measured as diameter — mean diameter.
Source: Wikipedia — Mars
Where it sits
Among the 16 planets & moons on this page Mars ranks 8th from the top by size. The largest in the group is the Sun at 1,392,700 km, which is 205× the size of Mars, while the smallest, Ceres, is 939 km.
Stand an adult of 1.7 m beside it and the ratio is 3,987,647×: you would need 3,987,647 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. Mars is 64,562× that, which puts it at 64,562 pitch lengths from end to end.
Light covers its diameter in 23 milliseconds. A jet cruising at 900 km/h would need 7.5 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 Venus at 12,104 km (39,709,974 ft), 1.79× the size. Both figures are the same kind of measurement, so the gap is a fair one.
The nearest thing smaller is Ganymede at 5,268 km (17,284,121 ft), so Mars is 1.29× as big as it. They sit in the same group.
Against fixed yardsticks
Against three fixed yardsticks — Mars is 20,542× the size of Eiffel Tower (330 m); Mars is 226,722× the size of blue whale (29.9 m); Earth (12,742 km) is 1.88× the size of Mars.
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 Mars?
6,779 km (22,240,814 ft), measured as diameter: mean diameter.
Where does Mars rank among planets & moons?
Number 8 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?
Venus, at 12,104 km, is 1.79× the size.
What is the next size down?
Ganymede, at 5,268 km, is 1.29× 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.