Outboard shaft length
Outboard shaft length is measured from the point inside the clamp where the transom would sit, to the bottom of the cavitation plate. A long shaft may have two plates, so it is naturally the lower of these. The rest of the lower leg, prop, etc. are not included in this measurement, which could run 8" to 10" depending on the specific motor. Your boat has a large cutout in the transom as a motor mount, does it not? If so, remember that your entire motor sits as much as a foot closer the actual hull than it would if it was on a lifting type bracket. This means that it is much less likely to ever come out of the water in rough seas than one on a bracket.
I know that there are those who say that you absolutely, positively must have a long shaft motor on any sailboat. I have actual real life experience that says otherwise. Way back when I had a Cat 22 I had a LS on a bracket off the transom, and even then it would indeed come out of the water in heading into big swells. Since then I have also owned 2 other boats that had transom cutouts and have used a variety of short shaft motors with absolutely zero problems in over 15 years and many, many miles of all sorts of waves, chop, swell and headwinds. Another benefit is that the leg won't drag in the water when sailing fast with the motor tilted up, as it surely will with your mounting point and a long shaft.
My advice would be a short shaft, but if you want a longshaft, find one and actually measure it carefully first.