Granularity can refer to the size of an individual sale, customer, or supplier in relation to the overall sales of the company. Understanding the granularity of a company's transactions helps the investor to assess the risk the company has in the market.
For example, Google is generally fine granularity, since it gets most of its revenue from small advertisers, and the loss of any one of them as a customer would not impact on the company. However they have made some coarse granularity deals, for example to display adverts on other major sites, where the failure to renew such deals would have a material impact on their revenue. One way of looking at the distinction is whether a deal would be considered a notifiable event to the stock markets.
Granularity can also be considered on the supply-side: if a company is reliant on just one supplier (the 'ultimate' coarse granularity of supply), and something affects that supplier's ability to supply, then the business is at risk.
Granularity is often related to the complexity of the product or service being provided; generally a fine granularity is associated with a 'simple' product (say a CD or a book) whereas a coarse granularity is associated with complex item (say a submarine). (The tendency of successful businesses to be based around selling simple products in large volumes is sometimes known as the 'Barbados principle', on the basis that many of the large properties there are owned by the founders of such companies)
Naturally a fine-granularity company isn't risk-free, since if all their customers are in the same sector, and that sector is affected by an external event, then the company will suffer from lots of seemingly independent, yet ultimately linked, purchasing decisions. The sub-prime mortgage market is an example of this.