There are several criteria to measure the size of companies revenue, number of employees, assets or market capitalization but for different causes any measurement can't be described as an accurate measurement.
1- Walmart 421.849 (billion $ -2011-) -USA-
2- ExxonMobil 370.125 (billion $ -2010-) -USA-
What is the biggest or largest companies in the world by number of employees?
1- Walmart 2.1 million -USA-
2- McDonald's 1.7 million -USA-
3- China National Petroleum Corporation 1.7 million -China-
What is the biggest or largest company in the world by capitalization in 2011?
1- Exxon Mobil 406,272 (billion $) -USA-
2- Apple Inc 376,410 (billion $) -USA-
3- PetroChina 276,844 (billion $) -China-