When I was about 11 years old we moved into a house that had a large prickly pear cactus in the front yard.. and my mom, being the can-do, pioneer woman she was said "I bet those pears would make a yummy jam!" ... so thats just what she did.. she made prickly pear jam that Summer... and it was, in fact,  yummy! 
Measurements are: 15.5" x 13.5" x 1"