Cellos are measured by the length of the back, from the full-sized cello of 30 inches or more intended for adults over five feet tall to 1/8 cellos designed for children between four and six years old. Be aware that different cello manufacturers make cello sizes at slightly different lengths, but they will fall within a few inches of each other.
The most important thing is comfort and practicality; you'll likely be more comfortable with the smaller instrument if you are between two different sizes.