Ask HN: market size -- how to estimate?

How do you estimate the size of a market? As a marketing/business newbie, I don't know how to go about figuring out how big a market may be, especially if there aren't already statistics and numbers already gathered. We might be able to find out how big the market is for small business accounting, for instance, but for more niche markets -- as a business development person, how would you go about figuring out a market size in size and potential revenue?