There's an interesting blog post here that sums up a debate in the US started last month when the Brewers Association (which represents "craft" brewers making up to 6 million barrels/700 million litres a year) issued a media release accusing big brewers of trying to hoodwink drinkers into believing some beer brands were craft beers when, in fact, they were made by some of the world's biggest brewers.
It's an interesting question.
(As a point of comparison to the US where a craft brewery is regarded as brewing up to 700m litres (recently upped from a third of that), according to Wikipedia, "in the UK, the term [microbrewery] has become synonymous with small scale breweries operating under the UK Progressive Beer Duty threshold of 5,000 hls" (i.e. 500,000 litres). As a further point of comparison, Coopers brewed 63 million litres in 2010-11, which would make it more or less a minnow in the US scene.)
Cheers,
Oliver