This paper examines the risk externalities stemming from the size of institutions. Assuming (conservatively) that a firm risk exposure is limited to its capital while its external (and random) losses are unbounded we establish a condition for a firm to be too big to fail. In particular, expected risk externalities’ losses conditions for positive first and second derivatives with respect to the firm capital are derived. Examples and analytical results are obtained based on firms’ random effects on their external losses (their risk externalities) and policy implications are drawn that assess both the effects of “too big to fail firms” and their regulation.