The size of a crime scene can vary greatly and the police must be prepared to quickly determine its boundaries. This task is not as easy as it seems. A crime scene may be a single room, an entire house, everything on a property, or even a whole neighborhood. And that's just the primary scene.
At a minimum, the crime scene includes
The exact spot where the offense took place
Areas from which the site can be entered and exited
Locations of key pieces of evidence, such as the body in a murder, a safe or cabinet in a burglary, or an entire structure in a suspicious fire
A crime scene can be cordoned off using crime-scene tape, barricades, automobiles, or even by police officers standing guard. Only personnel who are absolutely necessary for processing the scene are allowed in. This restriction often is more difficult to accomplish than you may think. A victim's family members or neighbors may be emotionally unstable and thus difficult to remove from the area.
And, of course, members of the press often have clever ways of gaining access to a crime scene, to say nothing of a captain or other high-ranking official trying to push her way past a lowly patrol officer who's following orders to keep everyone without a reason for being there away from the scene. Furthermore, you can never underestimate the meanderings of the curious bystander.
After the scene is secured, the first officer to arrive establishes a security log, which basically is a sign-in sheet that must be signed by any and all visitors to the scene. This kind of crowd control helps the investigation in many ways, not the least of which is limiting the number of people who must be examined when stray fingerprints and shoeprints are found. If investigators can be ruled out, the print or prints remaining may point to the perpetrator.
A crime-scene investigator begins by doing a walk-through examination, getting a feel for the scene and organizing an approach to collecting evidence. During this overview, the crime-scene investigator typically doesn't examine any particular pieces of evidence, but rather looks at the big picture before beginning the tedious work of evidence examination and collection.