I have built several dreadnought guitars over the years and only until now do have some decent photos to offer. The Dreadnought model originally came into demand beginning in the 1930s as a result of the growing popularity of bluegrass, country and folk music, which required a large-bodied guitar with a full, resonant sound. 

Dimensions
Upper Bout: 11 3/8" (28.9 cm) 
Waist: 10 5/8" (27 cm)
Lower Bout: 15 5/8' (39.7 cm)
Body Depth: 4 1/4" (10.8 cm)
or 4 5/8" (11.8 cm)