The length of the actual stop sign is 12 feet
What is a scale drawing
A scale drawing is a technique used to represent a shape or an object in a reduced or an enlarged size as opposed to its actual measurement.
Given that:
The scale of the drawing of a stop sign of 1 inch represents 1/3 foot, and the side of the stop sign represent 3 inches.
Then; the actual length of the stop sign is:

= 1 inch.
To feet, the length of the actual stop sign is 12 feet.