Information technology/information systems are currently used by almost all firms worldwide to integrate and coordinate their business processes, ranging from operational to strategic support. Among the systems adopted by firms, one can find enterprise resource planning, customer relationship management, business intelligence, and many others. This paper presents a study conducted with the participation of 102 Chief Information Officers (CIO) from Iberian firms, aiming to provide a better understanding of the influence of firm size on the adoption of the main enterprise information systems (EIS) types. The findings show that firm size influences the adoption of some EIS types as, for instance, business intelligence, workflow management and collaboration and groupware systems. However, in other cases, such as the case of enterprise resource planning, it no longer happens.