Asana Alphabet recommends yoga mats in bulk for schools--they are more economical and you can cut them to the actual size that you want (click on image to purchase).
Asana Alphabet recommended reading--click on a title for info/purchase:
Yogi and Yogette Learn the Asana Alphabet
Smart Moves by Carla Hannaford, PhD
Yoga Nidra by Swami Satyananda Saraswati
Theater Games for the Classroom by Viola Spolin
Yoga for the Special Child, Sonia Sumar
Or you may want to opt for a yoga blanket that can be folded into the size you want to use