First you set up your ratio: Length to height.
The Drone is 30/4 which can be equivalent to 7.5.
The prototype is 12/x which can be equivalent to 7.5 as well. Re-write your equation to solve for x.
x*7.5 = 12 ---> x=12/7.5. = 1.6. Now plug in 1.6 as x to test your equation so you get an answer of 7.5: 12/1.6 = 7.5.
So, the prototype for testing should be 12 feet long and 1.6 feet (19.2 inches) high.