This study of Unix file sizes was performed with the assistance of the Internet community in November 1993. An article was posted to Usenet requesting people run a simple shell script and mail me back the results. There is no such thing as an average file system.
Some file systems have lots of little files.
Others have a few big files. However as a mental model the notion of an average file system is invaluable. The following table gives a break down of file sizes and the amount of space they consume. The seductive power of mathematics allows us treat a 200 byte and a 2MB file in the same way. People I trust say they would have expected the tail of the above distribution to have been even longer.