I asked wikipedia, and this is what is said:
A microbrewery, or craft brewery, is a brewery which produces a limited amount of beer.[1] The maximum amount of beer a brewery can produce and still be classed as a microbrewery varies by region and by authority, though is usually around 15,000 barrels (18,000 hectolitres/ 475,000 gallons) a year.
take what you will form that, I really don't know whether it is accurate or not