Why are cells so small?

The second module in Biology is Cells.  One important topic is cell size or why cells are so small, essentially a surface area to golf each ratio.  How do you teach this?  My plan is to start with Candy Coated Biology and let the kids compare different sized M&M candy.  I hope I can find mega M&Ms.  My goal is to use Cell Size, Surface Area and Volume to drive home the idea that cells are small to increase the surface area for the transfer of ions, glucose, oxygen, and wastes across the membrane.  We made need to measure different cubes to understand the how bigger cubes have less surface area.  I’m going down to the basement to pull dice and boxes.