I think it is
1: 72
because there are 12 inches in a foot, and the room is 12 feet wide. 12 feet is 144 inches ( 12 x 12), and in the drawing, the width is 2 inches.
So, it went from its original 144 inches to 2 inches. I think it can also be written as 1: 72 because 1 is it's original from, and it became 72 times smaller.