How big is a king-size bed?

Roughly two meters long. That is 2.03 m (6.7 ft) long.

2.03 m long · 6.7 ft

Roughly two meters long.

Average person 1.7 m

King-size bed 2.03 m

King-size bed next to average person, drawn to scale.

Compared with things you know

A king-size bed is about 14× the height of a Smartphone Smartphone: 15 cm
A king-size bed is about 1.2× the height of an average person Average person: 1.7 m
A king-size bed is about 1.0× the height of a basketball player Basketball player: 2 m
A king-size bed is about 1.0× the height of a door Door: 2.03 m

Think you could size it by eye? A king-size bed is one of the objects in Sizedle.