Collaborate with peer-sized or larger-sized partner helps the firm to enhance its process, product quality, reputation, and market position. Therefore, when choosing collaborator, firms prefer peer-sized or lager-sized partners. Many empirical researches try to link the firm’s size with the performance and result of collaboration. However, there are still many debates. Instead of using the firm’s size, this paper use the compared size or size difference between collaborating firms to examine its influence on the performance of inter-firm collaboration. The results from qualitative case study and quantitative online survey in both Australia and China supported that size matters when firms select their business partners. Size difference also influenced trust level, the quality of communication, risk level, similarity in structure and process, and as a result, the performance of inter-firm collaboration.