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 Crescent Girth
Specifically designed to fit horses with a narrow front and bulging ribcage starting directly behind the elbow. Often to be found in short backed horses, (typical for Arab Mares) where other girths don't give enough room. The Crescent girth is cut in a way that it opens to the rear in the sides and below to accomodate the far forward reaching 'bulge'. Most of these horses do not have an appreciable or no girth groove at all.