Thanks for making such a helpful application. Here are some feature requests.
1. When the staff is disabled, hide it so that the guitar fretboard can be bigger (vertically). Also, reduce the space given to the staff in "Name That Note" mode so that the fretboard can be bigger (vertically).
2. When the fretboard is longer than five frets, make the higher frets slightly diminish in size to reflect a real fretboard. In "Play That Note" mode this might not be possible unless you make the sound hole a lot smaller, but in the other modes it would help make the fretboard look more realistic.