Great photo! Looks to be a pair of antelope and a Winchester '86.
Horse is at the end of the reins. So horse is closer than the people. Makes the horse look bigger and the people smaller. But one thing for sure that stirrup is long ways off the ground. I'd guess the stirupps are 4 to 6" higher in that photo than in my photo. Mine @ 6" lower? Are still a bitch to get a foot in

So I am thinking big horse (16+) and short people.
This is 16.3H horse and a 6'1" rider.
Same horse. Rider is 5'3". I'd need a BIG stump and some sort of a kung fu stretch just to get a foot in that stirrup