We made this at our Christian School, and the elementary-age students really enjoyed loved it!! They did the work, and it was fun.
About making larger cakes with this, if you keep the 3 (cake mix) to 2 (water) ratio, you could make any size. Like 3 cups cake mix and 2 cups water, or 3/4 cup cake mix and 2/4 or 1/2 cup water. Haven't tried this, but should work. You have to experiment with time to cook.