That length varies between individuals. The coffin bone is suspended within the hoof by its front and sides. It does not rest on its base. In fact, derotating the bone into that position can put the bony column within the digit out of alignment. It compresses the front of the coffin joint and opens the navicular area to direct trauma from below.
Most healthy hooves have the coffin bone slightly tipped-down within the foot.
The frog is not the blood pump some have claimed, and does not have to bear weight for proper hoof function. I hate to burst her bubble, but the Horse of the Future is not going to be a pseudo-mustang galloping across 30 miles of open country every day. Your horse is not a wild animal. He is the result of thousands of years of selective breeding to meet human needs. She never made a horse that could win the Kentucky Derby or a big 440 quarterhorse race.