Answer:
I think the actual length would be 75 inches. ( i'm not totally sure though C: )
Explanation:
So, since the scale factor is 1 : 12, the model is 1/12 of the actual. So, multiply:
6.25 * 12 = 75
So, the actual length would be 75 inches.
To check:
75/6.25 = 12!