There is a book called "Trails Manual" by Charles Vogel, published by Equestrian Trails Inc., P. O. Box 44135, Sylmar, CA 91342 in 1982. This book contains excellent guidelines for building equestrian trails.
The following illustration appears on page 63: (click photo for larger view)
This illustration recommends a trail tread width of 8' and a height of 10'.