Answer
The red sphere represents a metal and the blue sphere represents a non-metal.
Work Step by Step
Non-metals usually gain electrons when chemically bonding, making them 'anions' and electrically negative (because they have gained electrons and are now more negatively charged). Anions are larger than their parent atoms because they have more electrons. This would make the blue sphere a non-metal, because it has gotten bigger in the reaction. The red sphere represents a metal because it has gotten smaller. We know that metals usually lose electrons when bonding, making them 'cations'. Cations are smaller than their parent atoms.