Choosing the right size for a bokashi compost is completely dependent on the environment that it will be placed in. It's important to take into account how much food waste on average is produced, and how much space is available for the compost bin itself. The only limitation is that the container must be anaerobic (no oxygen) to house the bokashi process. I'm going to be living with 6 other people next year, so I figured a standard 18 gallon rubbermaid would do the trick. 2-5 gallon paint buckets are most often used.