Our small boxes fit in our large boxes (and need a single layer of our bubble between the boxes to secure them). In our experience a single box is sufficient protection, however this would be an even more secure way of sending them that was going above and beyond. You might want to consider this if you are shipping a guitar of high sentimental value.
 
The listing includes two boxes and enough bubble wrap to package the guitar and fill the small space between the boxes as well.

The internal measurements of the box is 16x44cm. It can be extended to as long as 140cm.