It does look impossible but I think it is suppose to. It's an incredible feat, one that even the Colonel and his soldiers are amazed to see. It was a brave act that most horses wouldn't even consider doing. The writers wanted Spirit to stand out from your "normal" horse. By giving him that task he stands above the rest.
In real life, I believe that act is impossible. The distance from the movie looks much further than 30ft. :)
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.