They don't _have_ to be identical, but it's a good idea.
As far as size goes, your RAID0 volume will be twice the size of the smaller drive, so it makes sense to match sizes.
Performance will be about twice the speed of the slower drive, not the average of the two. (Disclaimer: that comes from reading, not personal experience with benchmarking).
And finally, the WARNING: RAID 0 volumes are more than twice as vulnerable to failure than two single drives, and normal data recovery won't work worth excrement if there is a failure or corruption. Back up early, and back up often.