I would agree with the above poster in that no two crime scenes are alike, but I would also say there is a "typical" size for a crime scene. Crimes tend to be committed in isolated settings, that is, out of the public eye. This often means they are committed indoors and behind closed doors, which relegates the size of the crime scene to that of an average room - 12 X 12. The point of the actual crime itself - a shooting, an assault, a robbery - is quite small, a few square feet even. It is the scattering of evidence, blood spatter, DNA, shell casings, etc. that expand the size of a crime scene in many circumstances, and that scattering also has "typical" limits, whether it be from the walls of a room or the distance such evidence can travel.
No, there is not a "typical" size for a crime scene. The size of the crime scene depends upon the alleged crime that was committed and how many victims of the crime are involved. Some crime scenes can be very small in size, for example, the interior of a car or the inside of a small room. Other crime scenes can be quite large, say for example, a crime committed at a mall or school. The size of the crime scene encompasses where the illegal act was performed, where property was moved to(if applicable), or where the location of victims are. It is possible to have many victims in different locations that were all involved in the same crime and or committed by the same perpetrator or perpetrator's.
A basic tenet of law enforcement policy and practice is to secure the crime scene in an effort to ensure the safety of victims and police and also to preserve possible evidence for collecting that can be used at a later date to help prosecute the crime.