How large is the box in soccer?

Three are two different "boxes" in soccer: the goal box and the penalty box.

The goal box is the small box in front of the goal within which the ball must be placed to take a goal kick. (The ball can also be placed on the line). Size will vary by age group & your soccer club's rules. On adult sized fields the Goal Box extends six yards or 5.5 m from the Goal and Goal Line into the field and for this reason it is sometimes referred to as the "Six". According the official FIFA "laws of the game":

Two lines are drawn at right angles to the goal line, 5.5 m (6 yds) from the inside of each goalpost. These lines extend into the field of play for a distance of 5.5 m (6 yds) and are joined by a line drawn parallel with the goal line. The area bounded by these lines and the goal line is the goal area.

The penalty box, penalty area or simply "the box" is rectangular and extends 16.5 metres (54 ft) to each side of the goal and 16.5 metres (54 ft) in front of it. Previously, penalty areas extended the width of the field, but were reduced to their current dimensions in 1901. The penalty box includes the goal box. According the FIFA "laws of the game":

Two lines are drawn at right angles to the goal line, 16.5 m (18 yds) from the inside of each goalpost. These lines extend into the field of play for a distance of 16.5 m (18 yds) and are joined by a line drawn parallel with the goal line. The area bounded by these lines and the goal line is the penalty area.