Guitars come in many sizes and shapes but over the last century they have become somewhat standardized and the shape of a guitar is instantly recognizable and iconic. A guitar could be built that looks like a triangle a square or whatever, but we would not expect it to have the sound that we have become accustomed to as the sound of a guitar. A banjo works on the same principal as a guitar, but it doesn’t sound anything like one. So shape and size do matter, but if it is generally close to the accepted guitar shape and material, the difference is somewhat subtle.
We can make some generalizations about size and shape, but they can all be undone by other aspects of the guitar such as hole size or bracing or top stiffness or a lot of other factors.
From the smallest guitar to the largest jumbo guitar, they do not vary more than about 3” across at the lower bout and the great majority of guitars fall within an inch either side of 15” across the lower bout. The length of the body is similarly within a fairly narrow range, usually within an inch either side of 19” in length. Guitar depths are wedge shaped to prevent standing waves, customarily being deepest in the tail of the guitar. They generally are between 4” to 5” in depth at the tail with the majority around 4 1/2”.
Intuitively we might believe that a larger body might have more bass response than smaller bodies. That is partially true if one compares the extremes of the guitars from the smallest to the largest. The differences between guitars hovering around the average size are much more subtle. In my experience, the dreadnought size tends to have a little more bass response, akin to boosting the bass EQ on your stereo and the medium body sizes tend to have a more balanced response across the spectrum. Smaller bodied guitars tend to have a little more midrange to their response but have their own sort of sweet resonance.
It is a common assumption that the larger the body, the louder the guitar will be. I’ve found that is not necessarily the case. The medium and small body sizes can be just as loud as the dreadnought sizes. In some ways, the medium and small guitars tend to "cut" through the mix and have excellent projection.
In some cases, body shapes are chosen more for historical reasons. Most bluegrass and flatpickers tend to favor a dreadnought style and most finger style or folk players tend to favor a medium body size. There are always exceptions of course, I believe that some of the early James Taylor finger style playing was on a large body Gibson J45 and Eric Clapton has an Orchestra Model (medium body size) named after him.
Those who like to play above the 12th or 14th fret may consider adding a cutaway. Since the upper bout of the guitar is rather heavily braced to support the neck loading it does not greatly contribute to the guitar’s sound. I have not detected a discernible difference to tone by adding a cutaway although they do require additional building and fitting time which translates into higher cost.
It appears to me that everything about a guitar affects the other aspects of the guitar. Body size and shape is just one of the factors. Other factors such as scale length, number of frets clear of the body and wood choices are equally important.
The bottom line is pick a body shape that fits your body size, is comfortable to play and is aesthetically pleasing to you. Your luthier should be able to help you with some of the other decisions to get the most out of it.
Redd Volkaert's OM Cutaway Acoustic Guitar