Here's how to find the size of the data you're going to backup:

1. Right-click on the "Start" button.
2. Click on "Explore"
3. Right click on the folder that contains the data you want to backup, such as your "My Documents" folder.
4. Now click on "Properties".
5. On the "General" tab (selected by default), wait until Windows finished calculating all the size of your data, and then enter in the calculator the number (in gigabytes) that's displayed at the right of the "Size on disk" label.