acoustic guitar size

The modern full size classical guitar has a scale length of around 650 mm (25.6 inches), with an overall instrument length of 965–1016 mm (38-40 inches). The scale length has remained quite consistent since it was chosen by the originator of the instrument, Antonio de Torres. This length may have been chosen because it's twice the length of a violin string. As the guitar is tuned to one octave below that of the violin, the same size gut could be used for the 1st strings of both instruments.

Smaller scale instruments are produced to assist children in learning the instrument as the smaller scale leads to the frets being closer together making it easier for smaller hands. The scale size for the smaller guitars is usually in the range 484–578 mm (19-22.5 inches) with an instrument length of 785–915 mm (31-36 inches). Full size instruments are sometimes referred to as 4/4, while the smaller sizes are 3/4, 1/2 or 1/4.

These sizes are not absolute, as luthiers may choose variations around these nominal scale lengths;

• 4/4 650 mm (25.6 inches)
• 3/4 578 mm (22.75 inches)
• 1/2 522 mm (20.5 inches)
• 1/4 484 mm (19 inches)

Source : http://en.wikipedia.org/wiki/Classical_guitar#Sizes