The only time this question is typically raised is w/ RAID1 (mirroring).
Since you weren't specific, I will assume this is what concerns you.
As far as hardware based RAID1 is concerned, ANY two HDs will work. If they
are different in size, the only thing that happens is that the array uses
the smaller of the two HD sizes. IOW, essentially the difference in space
on the larger HD is lost. So there's no obstacle to using different sized
HDs, it's purely an issue of ecomomics. Ideally, you'd like to waste as
little HD space as possible, so the closer in size, the better.