This paper deals with bid-rigging in a first price auction when bidders face the risk of a prosecution following a denunciation. In this paper, we analyze how cartel bidders react when they face the risk of a denunciaition. In particular, we define the cartel equilibrium size level considering two situations. At first, we retain d’Aspremont and al. (1983) equilibrium criterion where a bidder decides to enter the collusion if and only if his cartel expected profit is higher than his outsider profit. In this situation, we prove that no collusion is possible when the risk of an individual denunciation is high. Finally, we relax this criterion and consider that any bidder decides to enter the collusion if his expected profit is higher than his profit without any collusion. We prove here it is necessary that collusion be large enough to be stable when the risk of an individual denunciation is high.