The drives don't have to be the same size for Raid0 or Raid1 MooseMan, but you'll only use the drive space on the bigger drive up to the size of the smallest drive. That's because the data is striped across the drives for Raid0, and mirrored for Raid1.
What you might be talking about is JBOD (Just a bunch of disks!), another feature used by many inexpensive RAID controllers that simply spans drives of any size together into one large logical drive.
I'm wary to do either Raid0 or JBOD for anything that I want to keep, like pictures, documents, music or videos. If you think you're sad when one hard drive fails, multiply that sadness by the number of disks in your Raid0 or JBOD array, because when one disk kicks the bucket, all your data goes bye-bye.
Of course, there are specialized applications that can benefit from Raid0's speed and capacity advantages, like audio/video editing or other disk-swapping intensive applications, but I wouldn't recommend it for most people. Since the original poster seems to be worried about backing up the data on the first drive, I'm assuming it's not data that he cares to lose...
You can read all about it at sites like
www.StorageReview.com.