Depends on how wide the canyon is. Horses can jump some pretty big widths. I think Man o Wars stride was somewhere near 28ft... And that was just his stride.
The scene was meant to bring across the point that Spirit would go to any lengths for his freedom and jumping a canyon portrayed that. I guess it's possible to gage the canyon distance by guesstimating the height/width of the objects around it (Spirit and his rider), but that's a whole lot of science for a cartoon. And I'm sure the canyon actually changes width with the clips in the scene, because animation isn't exactly reliable.
And those who were seen dancing were thought to be insane by those who could not hear the music