The simplest way to choose your rental truck size is to eliminate possibilities that do not exist in your situation. For example, if you have chosen to utilize a particular rental company you will only have three available size options: 10', 16' and 24'. In case you're wondering, 94% of people relocating with a rental truck choose their company based on price or geographical limitations. With that in mind we can assume that most people will have between 3 and 5 truck size options to choose from depending on which company is being used.
Small rental trucks should be used primarily for local moves. Only consider renting a 10'-12' truck if you have a studio or small one bedroom apartment with minimal furniture.