Cover a greased 9 x 13 x 2 cake pan with 5 cups of bite-sized pieces of rhubarb. Sprinkle 1 cup of white sugar evenly over the top of the rhubarb. Take a 6 oz. package of strawberry Jell-O and sprinkle the dry Jell-O mix over the top of the white sugar. Sprinkle an 8oz. bag of miniature marshmallows over the top of the dry Jell-O mix. Mix up a box of Duncan Hines yellow or white cake mix according to the box directions, and pour the batter over the top of the marshmallows. Bake at 350 degrees for 45 minutes.