What size blower would you need for 20x40 pool?

A good quality shop-vac should do the job. A true 3/4 HP machine is right. That said, it's difficult to tell what the real HP is.
The returns should blow out with almost anything. The main drain is a different story. However if you get any bubbles out of the main, you'll be fine.
 
It does not take very much to blow out lines. Even a small compressor will do the job. A 10 foot deep main drain would only require about 5 psi to blow out.

Example = $59.99 from
http://www.harborfreight.com/air-tools/ ... 95275.html

You can use an adaptor that will screw into the pump drain plug.
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Example = $ 2.95 from
http://www.amazon.com/Amflo-701-2-npt-t ... B0002V4Q4I

Note: A Shop Vac probably won't blow out a main drain, as it is not designed to provide any significant pressure.
 
I think they are looking for a large volume of air, not that it really does anything better. Maybe it's just a mental satisfaction thing when you get lots of air coming out a return vs a constant burping that you'd get with an air compressor. I use the cyclone vac 99% of time, I use an air compressor on those paramount automated self cleaning pool things.
 
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