If you go here you’ll see an example of a reproduction of the M1908 cavalry era blanket that was used until 1948. http://onlinemilitaria.net/products/3028-US-M1908-Saddle-Blanket-new/?bc=no The Civil War era blanket was 84 x 72 and the '08 was slightly larger, IIRC. A Pendelton twin is 66 x 90, so it would be very close.
My photo collection is pretty sparse on blanket photos. I have precisely one of me and a few from older sources or modern competitions.
Attached is a good one that was sent to me by a friend. You can see the general fit under a McClellan saddle with saddle bags.
http://s784.photobucket.com/user/Smile225/media/Bigbear02-1a_zpsxqyl8dnr.jpg.html?sort=3&o=0
I found precisely ONE photo of myself using the folded blanket. Most are from competitions where I use a more formal pad.
http://s784.photobucket.com/user/Smile225/media/Occupation%20of%20Vera%20Cruz%20%20Small%201914_zpsx6awisk6.jpg.html?sort=3&o=12
During formal competitions during the Cavalry era the blanket under the saddle was routinely covered by a shabraque to give a more formal appearance.
If you trail ride you can use it as a “rug” if you modify it with some sort of attachment system. That system would have to be removable when the blanket is re-folded.
You can spend a LOT of money on saddle pads and not be satisfied. I’ve found the blanket folded cavalry style is a very effective solution.
G.