STEP 1. First you have to boot into the Mac OS X Installer CD/DVD.
STEP 2. Now you have to click through the steps until you get “Select the disk where you want to install Mac OS X.” As you can see from the image posted below.
STEP 3. Now you have to choose the drive and then click on Customize button.
STEP 4. Now you will be able to see the following window, as you can see several different installation options are available, You have to install Mac OS X with only the bare essential system software and uncheck all other options. Then continue through the installation.
That’s it. In this way you can install Mac OS X with only 6GB disk space. If you found any difficulty in following the above steps then feel free to ask in the comment section below. [Via]