If you look around the standards ask for 2 of the same size drives but if you insist some will do that and waste space. This might be "unacceptable" to new RAID users. That is if you RAID 0 a 500 and 250GB set of drives the result is a 250GB RAID 0 drive. Where did the other 250GB go? It's still there but not usable.
RAID would not apply to half a 500GB drive and not the other half. It's all of nothing.
Now you may find some roundabout off the beaten trail way, but those that go down that road are often upset at how lonely it gets.
I have a 500 gigs ssd in my computer, and a similar one , but 250 gigs.
Is it possible partition my 500 gigs ssd in two 250 gigs ones, and raid 0 one of them with the other 250 gigs ssd?
so i could get a 500 gigs faster ssd, and a 250 normal one