Hatpins were at their peak of popularity during the Victorian era, but as with many customs Victorian, are making a come back!  They currently vary in length from 6 to 12 inches, although they have been known to extend a full 14 inches  depending on the size of hat they were being used to secure! 

There are currently both American and British hat pin societies.  For a fascinating history of hatpins and other articles of interest on the subject, visit:

The American Hatpin Society site at:
http://www.americanhatpinsociety.com/tour/history.html

and The British Hatpin Society at:
http://www.hatpinsociety.org.uk/history.htm