To build a plane that is twice as big, you multiply every dimension by the square root of two. This is 1.414. If you multiply every dimension by 2 you will have a plane 8 times as big as the original. You must also include the height in the formula for the volume.
Do you see the contradiction? For the first statement to match up with the second you would increase the size by the cube root of two. So for an airplane with double the volume you increase the linear size by 1.260, however if you want double the area, then 1.414 is correct.
Nearly all the drag is created by the wings. The fuse contributes a lot of parasite drag.
If you actually do a numbers work up, you would find that the wing only contributes about 30% of the total drag in un-accelerated flight. A fuselage contributes about the same. However if producing a large amount of lift (high g turn or slowed up for landing) then the induced drag adds a significant amount of drag to the total.
If you fly at 50mph with the original size plane and then double the size using 1.414, the drag will increase 4 times if you fly at 50mph. With an increase of 4 times for drag, you will need 4 times the power to fly at the same speed.
Actually the drag is less than 4 times as great due to better Reynold's numbers due to the increased size.