Yes it's recommended you use the same size of drive.
Performance gain in minimal with two drives. Example, I have an array setup as RAID 0 and gain 7MB/sec (43MB/sec compared to 36MB/sec).
RAID 0 - Known to us common folk as striping. You take two or more drives and it will write (for example) a 128k file at 64k x 2. Basically it will take that 128k file and write half to one drive and the other half to the other improving write performance by half. Does that make sense?
RAID 1 - Also known to us common folk as mirroring or duplexing. This is self explanatory. It makes a copy of the data to the other drive in the array thus making it fault tolerant. **Not intended for speed.
RAID 0+1 - Combines both of those above and requires 4+ drives in the array.
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