Globalization opens new opportunities also for small and medium sized enterprises (SME). However, empirical information on SMEs' international activities is still fragmented. The paper adds another piece of evidence in analyzing how Berlin SMEs are co-operating with partners in transition economies. To identify the size impact, co-operation activities of SMEs are contrasted to those of large enterprises. It is explored where SMEs identify sources of comparative advantage, which mechanisms, which sectors and which regions they prefer to co-operate and which impediments they face. As a result, the impact of enterprise size often appears to be modified by sector adherence.