What resource is useful for determining the size of an endangered area during a hazmat incident?

The correct answer is the Hazmat resource guide, as it provides critical information regarding the potential impact area based on the specific hazardous material involved. This guide often includes details such as chemical properties, potential dispersion patterns, and environmental factors that could influence the size of the endangered area during a hazmat incident. This information can be vital for responders to assess risks to public safety and inform evacuation or protective measures.

While the other options may contribute to overall incident management, they do not specifically focus on determining the endangered area's size in the context of hazardous materials. An incident command system is designed to facilitate the management of resources and personnel during emergencies but does not provide detailed chemical-specific data. An emergency operation plan outlines the overall response strategy but lacks the precision needed for size estimation of impacted areas. A GIS mapping system can be useful for visualizing geographical data and tracking incidents spatially, but it requires accurate data input, which the Hazmat resource guide directly supplies for hazardous material scenarios.