As far as I know, and as supported by this article, you can setup a RAID array of different size disks, but the array will end up sized as Number of Disks * Smallest Disk.
Can I create an md(4) RAID array out of block devices of unequal size?
Yes, but the RAID array will have the size of the smallest block device (plus some overheads for its own housekeeping). If device sizes aren't within 1% of each other, you get a warning.
This concept also applies to hardware RAID controllers like HP's SmartArray line.
The article referened above shows you how to test the theory by creating dummy devices and then layering
md devices on top.