CO2 cartridges are available in various sizes, 12g, 16g, and 25g, depending on what you want to use it for. For small bikes, smaller 12g cartridges are ideal, while 16 - 25g cartridges are better for mountain bikes. However, the easiest way to know which size to opt for is by knowing the working pressure of your bicycle tire. Each cartridge comes with a working pressure and should match what your bicycle tire needs for full inflation.