Choosing the correct type and fit of girth means your saddle will be stable not cause problems for your horse. Girth shape, the amount of elastic, the placement of elastic, and where the buckles end when your girth is tight, all influence the fit of the saddle. For more discussion of girthing and girth types see "Tack in Focus" on this site.
A note on our girths: If the girth you've received is stamped a different size than what you've ordered, please measure it buckle to buckle before use. Some of our girths run large, so we will measure the girth before sending it to you to ensure that you receive a proper fit.