If I understand the way that RAID works, each drive needs to be the same size. So, like Drakino said, you can simply add more same-size drives to the SCSI bus and tell the RAID controller to go use them. That's assuming hardware RAID on a SCSI system.

Hardware RAID is very nice. On the RAID system I just got up and running here, the controller (the one built into the Dell PowerEdge servers) did a fantastic job when I deliberately yanked a drive just to see what it would do. The management software politely informed me of the situation. Then, when I re-inserted the drive, it silently began rebuilding the replaced drive without stopping service. Took 8 hours to rebuild the drive, but hey, it's better than failing completely.