"1 pint blueberries, 1/2 cup sugar. Mix and spread in pan. (What size I ask?). Mix any cake mix according to the directions on the box. Pour over the blueberry mixture. 1/2 of the cake mix. Bake at 375 degrees for 45 minutes. Serve with whipped cream."
My thought is that she used an 8" x 8" baking pan...hence using one half of the cake mixture.
Try it. Enjoy it.