Cloud Computing is not really a market or set of markets at all. It’s part of a paradigm-shifting trend that is reinventing how organizations large and small purchase, provision, utilize, pay for, and think about IT resources. Eventually, everything will be the Cloud, which from the market sizing perspective means that nothing will be the Cloud.
Group vendors into separate, distinct markets (or do so for individual products, when vendors compete in multiple markets). Assign each of your analysts to interview each vendor once a year in the analyst’s assigned market, in the hopes that the vendor will reveal their true revenues for the year in question. Put the resulting numbers in the spreadsheet, add them all up, and you are able to calculate a reasonable estimate for the size of the given market.