...Have you been to Japan before...? I don't think this is the sort of thing the Japanese collect statistics on... you're going to have immense trouble finding the information you're after. Maybe check the major Konbini companies' websites to see if they've published any financial data or accounting reports?
Konbini basically stock what you'd find in larger petrol stations here in Australia - magazines, newspapers, confectionary, drinks, sanitary items, batteries, instant noodles... you get the idea. They're also overpriced, just like petrol stations here - you pay extra for the convenience of the store being close and open late at night.
In addition, you can also purchase hot food, hot drinks and obentou at most Konbini.
In Tokyo (not sure about the rest of Japan), you can also purchase garbage bags from konbini - garbage collectors in Tokyo will only take rubbish that has been put in their branded green bags. These bags cost more than normal plastic bags, because the cost of rubbish disposal has been incorporated in the price.
The main brands of Konbini are Family Mart, Lawson and 7/11.
Konbini also have ATMs. Often, these ATMs accept foreign bank cards - VISA, Citibank, etc.
You can pay utility and phone bills at Konbini.
You can also buy stamps and postcards, and some Konbini also offer delivery services.
Sometimes they have toilet available for customer use, and sometimes they have a photocopier.
You can also purchase revenue stamps at Konbini. Revenue stamps are used for paying fees to the government. For instance, I needed to purchase revenue stamps for my multiple re-entry permit application (for my student visa).
I guess both an upside and a downside to Konibini is that there's so many - there's bound to be one nearby, but it's kind of like seeing a McDonalds on every street corner here in Australia.