A person on some forum posted that you cannot use the extra left over space.
I've reposted his answer here incase icrontic disappears before serverfault does:
Could I partition a second larger drive with one partition the same size
as the first smaller drive and a
second partion with whatever space is
left, and then RAID 0 the first drive
and the matching partition of the
second drive. Then, could I have a
left over partition as an extra
No you couldn't. When a RAID
controller downsizes a drive all
remaining space is gone and not
accessable as long as the drive is
part of an array. It does this because
the drives are not spanned as everyone
thinks. They are striped together. In
otherwords woven together. If you
write a 32Kb file to a 2 drive RAID-0
with an 8K stripe it writes 8K to
drive 1, 8K to drive 2, 8K to drive 1,
and 8K to drive 2. So 32K is written
in 4 alternating chunks to 2 drives.
Therfore both drives must exactly
match on space.