The march really was historically big, but history also shows numbers don't guarantee success.
An examination of marches in American history reveals a mixed record of success, often delayed. It took 20 years to see any results from the massive 1987 gay rights march, for example.
Here's a neat map of marches, protests, and rallies that have taken place in Washington over the past century, that gives some context on size. The March on Washington was smaller than Saturday's march, while the Million Man March was bigger.