A common question that I'm often asked is about the different body sizes when choosing the right acoustic guitar. While not an easy question to answer, there are some basic guidelines to consider.
The first thing is determined by your playing style and the type of music you play. For instance, a Bluegrass player will typically want a full size guitar like a dreadnought. This is of course is not a 100% answer, because bluegrass players still play a variety of body shapes and sizes. But the dread is certainly the most common.
Now if this is a guitar that will mostly be played at home and sitting on your couch, then a small body guitar like a Parlor or what's often referred to as a single 0 may be the better choice for you.
If you are a finger style player, then a guitar with a faster response and more articulation is likely the right choice. This would lead you to an orchestra or concert body. Sometimes this will include what's called a 000.
As a side note, the number designations like single 0, 00, 000 etc. all have their roots in Martin guitars history. These are machinists number and indicate the size starting with the single 0 as the smallest and moving up to larger body sizes.
It's always best to put these different size guitars in your own hands because everyone is different. And the body size is only one small part of the equation. Other factors like nut width, neck profile, scale length, string gauge and woods will all combine to make each guitar very individual. And the one that's right for you is not especially the one that’s right for the next person.