This would be a total guess, but I'd say they'd consider anyone earning less than $5,000/month a small publisher, maybe even $10,000/month.
It's evidently 20 million impressions per month, as that's when they feel you're important enough to be a "premium publisher".
When you want to target a site in adwords, there is no difference between 0 and 9999 impressions. Sites are sorted by impressions and the benchmarks are 10k, 100k and 500k per day. The system uses those benchmarks to tell advertisers how big a site is. tiny = less than 10000 impression / day, because that´s all sites with 1 to 9999 impressions. Estimated max: 1000 usd / month small = between 10k and 100k impressions 100 Estimated max: 30000 usd medium = between 100k and 500k Estimated max: 100000 big = 20 million or more than 500k per day Estimated max: no limit and of course the cpm drops when you get more visitors. Becasue more visitors = more repetat visitors = more ad blindness.
Well, theres nothing called Small publishers or Big publishers.. Adsense is open to all. Though they have a group called Premium Publishers.