This pair of leather and brass postol holsters measures around 38″ overall, and each holster around 11″. A pair like this would be placed on the back of a horse, the rider having easy access to the pistols if needed. For source and more details, click here.
Here is another example:
Image from: http://www.cr.nps.gov/museum/exhibits/revwar/image_gal/morrimg/holsters.html, http://www.nps.gov/history/museum/exhibits/revwar/image_gal/vafoimg/frameshell.html