Measurements are from buckle to buckle. It is imperative that the girth be long enough so that the ergonomical features (the elbow relief area) are utilised. 
 
How to measure your horse: using a soft dressmakers tape measure, find the central groove in your horses sternum (area where the girth passes under your horses belly in line with the girth groove) and place one end of the tape measure here. Bringing the tape up firmly against your horses side, measure to approximately 5-7 inches above the point of the elbow. This will vary based on the size of your horse: the larger the horse, the higher you measure to. Then multiply this measurement by 2 and select the size girth closest to your measurement. It can be helpful to put your saddle pad on your horse. You will find that the buckles of your new girth will only be a couple of inches below the bottom of your pad (see photo example). Do not worry if you only have a couple of holes left in your girth billets when your girth has been fully firmed-up. This is desirable and really quite necessary, otherwise the elbow relief area will be too low and the ergonomic benefits lost.
 
The Athletico Girth
This girth has a specific fit adapted to horses with an athletic, wedge shaped body.  In a way it addresses the reverse problem that the crescent shaped girth solves. The saddle has a tendency to slip backwards, also pulling the girth backwards, because all the lines of the horses body:  back, ribcage and sternum to belly, taper towards the rear. So instead of opening to the rear, this girth simply opens to the front, to a degree. Naturally the internal construction is essential for the function of the girth. We developed this design because specific problems were pointed out to us by several riders and horse owners on quite a few jumpers and specifically in eventing horses.