The Acoustic Body

Parlor

With a few exceptions, the parlor is the smallest acoustic guitar body size. Measuring about 13 inches at the lower bout, parlor guitars generally also have necks that meet the body at the 12th fret. Parlor guitars tend to have an intimate voice perfect for light fingerpicking, and because they're comfortable to hold, they are an excellent choice for smaller players.

Classical Size

Based on the the design of Spanish luthier Antonio Torres in the mid 1800s, classical guitars are similar in proportions to the parlor yet considerably larger on the upper bout with deeper depth. Classical guitars are generally smaller than the more modern steel string acoustics.

Their compact proportions have a purpose of course: since proper classical playing position dictates the body and neck to be angled upward, the size of the body needs to be a manageable size, able to fit in a pocket between the players chest and knees. 

Concert (O)*

Measuring about 13½" at the lower bout, O-size guitars were considered large when they were introduced in the mid-19th century. Today, they are an excellent bridge between a parlor - which some players find too small-and larger guitars, offering great playability and intimate tones.

Grand Concert (OO)*

Typically measuring about 14¼" at the lower bout, the OO's larger dimensions produce a more muscular voice that works well for a variety of playing styles. This size is often used for acoustic-electric guitars because it offers a balance between good acoustic sound and controlled resonance when amplified.

Auditorium / Orchestra Model / Grand Auditorium (OOO/OM/GA)*

OOO-size (aka OM for Orchestra Model or GA for Grand Auditorium) guitars measure about 15" at the lower bout. This size has become the blueprint for many "small bodied" guitars and produces a great balance between volume, tonal balance and playing comfort. The OM designation was pioneered by Martin and is identical in body size to a OOO, but with a slightly longer neck and wider nut (typically 1¾" instead of 1¹¹⁄₁₆" of the OOO and GA).

Dreadnought

Introduced by Martin & Co. in the 1920's, the Dreadnought is the most popular body shape of all and is THE perennial bluegrass favorite. Surpassed in size only by the Jumbo, the Dreadnought was named for its large size after the British battleship H.M.S. Dreadnought. It measures 15⁵⁄₈" at the lower bout with a waist that's less pronounced than other acoustic body shapes.

Dreadnoughts are also typically quite deep, resulting in a lot more volume and a meaty, muscular voice. And while this size guitar is suited for flatpicking, the dreadnought can be quite versatile, and many players use it for strumming and fingerstyle playing alike.

Jumbo

Measuring about 16¹¹⁄₁₆" at the lower bout, the Jumbo guitar was introduced by Gibson Guitar as the extremely popular J-200 model in 1936. Inspired by the singing cowboy movie star Ray Corrigan this guitars has been less defined in its' tonality but more so with the country and rock-a-billy stars that were playing them.

Jumbo's are perfect strummers, project loudly and are a great guitar for sitting around a campfire. Guitar-makers use the body style as a starting point for lightly braced finger-style guitars.