Universities have created USOs to exploit the research knowledge and contribute to the economic development of their regions in the last decades, leading to an extensive literature on the topic. However, this growing literature has widely overlooked the links between firm size and survival. This paper explores simultaneously the role of size and other firm characteristics on the likelihood of the USOs’ survival, mainly drawing on the RBV of the firm. The empirical study uses an unbalanced panel consisting of 2,220 observations from 465 Spanish USOs observed between 2005 and 2013 and event (survival) analysis techniques. The results confirm than firm size is positively associated with the USOs’ survival. Moreover, the empirical evidence seems to support the existence of a minimum size that, once reached, makes the failure risk of USOs not significantly dependent on size itself. The findings also confirm that the determinants of survival consistently differ between micro USOs and SML USOs. Thus, the survival of micro USOs is negatively affected by those activities that involve high needs of resources, like patent activity or debt payment. In contrast, exporting increases the survival probability of SML USOs