You can easily resize any disk partition in Mac OS X using the included Disk Utility app, located in /Applications/Utilities, and you can even resize a mounted volume. In fact, you can grow or shrink HFS+ (Mac OS X) Partitions on the fly, with the resizing done live regardless of whether the drive is internal or an external drive, or even the boot volume.
A word of advice: Resizing mounted boot volumes isn’t always the best idea in the world because there’s a tiny potential something could go wrong. Before you go messing around and resizing your partitions, be sure to backup your data using Time Machine or whatever other backup service you prefer and make a recent back up.
Resizing a Mac Drive Partition with Disk Utility
Open Disk Utility and select the hard drive with partitions you wish to resize
Select the “Partition” tab and then click the partition to resize, then click and hold on the little dragging widget in the corner
Resize Partition in Mac OS X with Disk Utility
Size the partition as you wish, then click “Apply” for the changes to take effect