The easiest way to understand these ratios is to take a cube of side a. Its volume is a^3 cubic units.
Its surface area (6 square faces) is 6a^2 square units. The SA to V ratio is: 6a^2/a^3=6/a.
If a=2 (2X2X2 cube), the ratio is 6/2=3. This is measured in units^-1.
If a=3, the ratio is 6/3=2 units^-1.
If a=1, the ratio is 6 units^-1.