Not an hint by my own, but worth sharing it I think. Take a look at Make an Encrypted Sparse Disk Image over on themacmind.com. This explains how to use the command line disk utility (hdiutil) to create an encrypted disk image that requires only as much drive space as required by whatever is on the image.
[Editor's note: If you create a disk image in the Finder using the Disk Utility application, it will take up as much space as you allotted it, even when empty. Using the command line as explained in the article, you can create an image that grows as required, though it will not shrink when you remove items from the image.]
Mac OS X Hints
http://hints.macworld.com/article.php?story=20030212055706937