There are very long western saddles. I've seen skirts of 28+ inches with square corners advertised. Those would interfere with the hip on a shorter horse. There is a reason all my saddles have rounded skirts. My biggest horse was Mia, and @SouthernTrails
had the makers shorten the skirts of her saddle to under 26 inches.
Still...Bandit in my Australian saddle (click on to enlarge):
I'd bet money it goes past T18. Not by a lot, perhaps. But it also curves up in the back, and I think that made a difference. My Bates Caprilli AP saddle, unlike the CC version, was very flat in the back. It is one of the reasons I stopped using it. I could both feel Mia's back shoving against the rear of the saddle, and see it from the dirt/sweat marks on the saddle pad - unlike both my CC saddle and Aussie. She would get rub marks at the loin if one looked closely. That didn't happen with my Aussie saddle although their lengths were identical.
And of course, much of my
riding involves less motion of the back. We are not jumping or 'collecting'. Lots of walking, some trotting and cantering - but almost never for more than 300 yards due to the terrain. OTOH, I don't know if I ever ride at less than 25% of my horse's body weight, which some English shows are banning as "cruel". Overweight riders asked to dismount at show - Horse & Hound
Like everything else involving saddle fitting (and riding), there are so many variables involved that hard rules tend to be easily misapplied. That doesn't mean one shouldn't THINK about them. Just not use them as absolutes.
BTW: 13.0 hand Cowboy. We did 3 hours that day and he was willing to break into a trot on his own initiative even at the end of the ride: