As our dependence on complex critical infrastructures grows, the need for these systems to be robust, sustainable and resilient becomes ever more crucial. At the same time, an efficiency/economic imperative exists which tends to drive these interconnected systems to ever-larger sizes which then demands the question: Is the continuous increase of the size of critical infrastructures beneficial to our society? Because catastrophic failures of these complex infrastructures have enormous human and economic cost, the increase in potential cost of failure as the size increases must be taken into account when analyzing these network-based systems. With ever growing networks in critical infrastructure systems, the impact of network characteristics such as size and homogeneity on their complex system dynamics and risk of failure becomes crucial. A dynamic model of one such system, the power transmission grid, is used to investigate the risk from failure as a function of the system size. It is found that there appears to be optimal sizes for such networks where the benefit given by increasing the size is balanced by the risk of failure. Beyond this size, the cost of these rare large failures exceeds the gains in efficiency of service given by the larger system size. This is likely generalizable to other infrastructure networks with complex system characteristics such as the internet, general transportation networks and even economic markets and could have significant implications for how best to design and operate these systems.