Question: I currently have 3 1TB drives, a couple 500GB ones and some 750GB ones. Can I put them all in a RAID 5 configuration or do they need to be of same size?
Answer: There are a couple of methods to do this.
1) Use every drive as a separate node in the RAID-5 array. Only 500GB is used in every disk, the other space is wasted. This is typically the only option in a hardware raid controller.
2) Use a software RAID system combined with a logical volume system like Linux LVM. Using LVM, you can create a virtual drive of 1TB by combining two 500GB drives. This virtual drive can serve as a single drive in your RAID-5 array. Once one of these drives fails, the whole virtual drive is considered faulty. So please note you will need 3 virtual disks of the same size.
3) Use a RAID system that allows redundant storage on non-equal volumes. ZFS is a good example of this.