This is the part I'm most highly interested in. I have heard people talking about size differences between compound turbos of as little as 20% (sizing the little one at 90% and the large at 110% of a single turbo flow), this is the first time I've heard of making the big one double.In the very nature of compounding the first stage must be larger than the second on the order nearly equal to the pressure ratio it delivers to the second stage. (In the example I gave it would need to be around twice as "big" or more depending on other conditions)
Of course it will depend entirely on the application. When I get around to trying it my goal is boost at low rpm's and holding through to the highest rpm's you use in normal driving. I'm after maximum drivability and no smoke.
So for me the larger turbo will only be sized big enough to flow the expected maximum airflow.
Quite different to making a dyno queen where the only concern is power. This is always going to lead you to bigger turbos for their lower exhaust backpressure and hitting good efficiency at the engines maximum flow.