My approach would be to partition the disks in similar sized partitions, and create RAID1/RAID5 across them, and then use LVM to collect them as 1 device. Example with 4 disks:

Code:

+----------+
| 1TB      | Disk 1, 1 TB
+----------+
+----------+-----+
| 1TB      |.5TB | Disk 2, 1.5 TB
+----------+-----+
+----------+-----+
| 1TB      |.5TB | Disk 3, 1.5 TB
+----------+-----+
+----------+-----+-----+
| 1TB      |.5TB |.5TB | Disk 4, 2 TB
+----------+-----+-----+

The 4 partitions sized 1 TB would be made into 1 software RAID5 with a total usable size of about ~1.3 TB. The 500GB partitions would be assembled into a RAID5 with about 1 TB total usable space. With an LVM across both RAIDs you'd be looking at ~1.8TB total space that's protected from disk failure. The left-over 500 GB could be used for temporary space, added to the LVM as snapshot space, or to hold the OS.