The fit of a Scythe Supply snath is based on three measurements: height with shoes on, hip to ground (femur bone socket to ground measurement), and cubit (elbow to finger tip measurement). Before gluing the handles, try the handle position with a tight fit: place a thin cloth over the tenon and stem ends and wedge the handles on. Try mowing. Make adjustments.
With your assembled scythe test its fit by holding it next to you as in the photo above. The lower handle should come to about the peak of your pelvis (several inches higher than your hip to ground measurement, right around your belt), and the upper handle somewhere around your chin. And if you put your elbow on the lower grip and reach to the upper handle it should be 1-3" out of reach. Some variation is fine, the main point being mowing comfort. If your mowing stance feels comfortable and you don't have to stoop to make the blade lay correctly, then the fit is fine. In use the blade should lay with it's bottom on the ground with the edge about 1/8" to ¼" off the ground.
Once you are satisfied with your fit and comfortable with your handle positions you are ready to glue: mark the handle positions, remove cloth, reposition handles and line up with marks, glue with wood glue (TiteBond II recommended).
For more info on snath fit click here.