Thousands of radiation detection systems have been deployed over the last decade to domestic and international locations, primarily at points of entry or departure. An essential ingredient in supporting sensor deployment is the capability to develop and test emerging systems and replicate the behavior of deployed systems. Pacific Northwest National Laboratory recently designed and constructed a laboratory for developing and testing these large-scale radiation detection systems. The Large Detector Laboratory is capable of housing up to 30 fully integrated radiation portal monitoring systems and allows for complex testing scenarios that include background suppression, equipment shielding, and source cross-talk between systems. The laboratory's design also allows implementation of applications ranging from pedestrian screening to baggage or package inspection to vehicle and cargo inspection to standoff detection. This manuscript describes the unique attributes of this laboratory, including the ability to reproduce operational conditions faced by integrated systems, realistic source configurations, and measurements across multiple systems.