Command-line utility to capitalize long words in a file
As a Command Line Utility

The Argparse module is designed to facilitate the creation of command line utilities. It makes processing command line arguments much easier, and will even automatically display usage information if the user incorrectly enters arguments.

Potential Flaws

Your program does not preserve whitespace. This could be by design but I suspect it is not.
Example:  


the      quick      brown  fox


will become  


the Quick Brown Fox