Initial size-up of an emergency incident is conducted by the:

The initial size-up of an emergency incident is primarily the responsibility of the first-arriving unit. This includes the first engine or truck company that arrives at the scene of an incident, which is generally executed by the first firefighters on the scene. The size-up involves assessing the situation to determine the extent of hazards, the nature of the incident, and the resources needed to effectively manage the emergency. This evaluation is critical for establishing an action plan and ensuring safety for both responders and civilians.

While the reporting party may provide information about the emergency, their observations are not as comprehensive and tactical as what the first-arriving unit can assess on-site. Similarly, a first-arriving chief officer conducts a size-up, but this typically happens after the initial unit begins their assessment; their role is more focused on overseeing operations rather than the actual preliminary size-up process. The telecommunicator’s role involves conveying information but does not include assessing the situation firsthand. Therefore, the first-arriving unit is best equipped to perform the crucial initial size-up.