I love this mdadm-stuff, as it adding a huge benefit for linux (and of course for myself). You are talking about future-capabilities, which I would love to see: "If we did have support for variable drive sizes in a RAID5"
My server has actually a raid6 of 6 active 500GB-drives and 1 spare. I would like to add another 1.5TB-drive, but I do not like to split this new drive into 3 * 500GB, as then failing this single 1.5TB drive would render my raid unusable. I have seen ideas of striping e.g. the existing 500GB-disks to a 1-TB-disk, so I could assemble my raid6 with a new 1 *TB native and e.g. 3 * striped 500GB (=3 * 1-TB-disk). However this would result into a be degraded raid6 as well, unless I add 3 *1TB to this raid.
So having the capability of mixing different sizes in a raid6 would be genious. But I guess there would be limitations, as of the size for the spare and I am not sure what would happen if the largest-drive would fail.
However, even with todays flexibility, this mdadm is an outstanding useful tool! congrats!