What is the best combination of HDD size?

For gaming? Either get an SSD, or drop the AID0 idea. The money is better spent somewhere else.

I hope you are running a hardware raid config instead of the built in software raid on your mobo. The software tends to not be stable over a period of time.

If you need more space I would go with the 2x 1tb. Unless you have money to burn I would just go with a single 1tb and forget the raid or go ssd as suggested above.