How to Choose? I usually recommend starting with your physical size. If you aren't a big person, you will likely be more physically comfortable with a smaller guitar, which narrows it down to either the OO or Orchestra Model. The OO is the smallest of the two, and makes for a very comfortable and intimate guitar to play. Its sound is similarly sweet, intimate and focussed.
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For a larger tonal spectrum that affords a little more access to the lower frequencies, consider the OM or Grand Auditorium. Between these two models, the Grand Auditorium tends to produce slightly more bass, and the OM tends to produce a perfectly even balance, but the differences are very subtle.

Grand Auditorium

Width at lower bout: 16"
Width at upper bout: 12"
Length of body: 20"
Depth of body at endpin: 4 5/8"
Depth of body at neck: 3 3/4"​

Orchestra Model

 Width at lower bout: 15 1/2"
Width at upper bout: 11 1/2"
Length of body: 19 1/2"
Depth of body at endpin: 4 3/4"
​Depth of body at neck: 3 3/4"

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Width at lower bout: 14 5/16"
width at upper bout: 10 7/8"
Length of body: 18 7/8"
Depth of body at endpin: 4 1/8"
Depth of body at neck: 3 11/32"
I will accommodate special requests for the following dimensions, but these are my standards:
scale length: 25 3/8"
width of neck at nut: 1 3/4"
string spacing at bridge: 2 1/4"