i had an OS 1.60 twin which is the same weight as the DLE 30, and it balanced perfect, so give consideration to that before deciding on the 20.
sure the 20 will fly it, but not much better than the 1.20 four stroke that GP recommends.
take a look at the original GP Super Stearman thread to see what other used for power and found the plane to be much responsive.

i'm not knocking the 20 as i have a DLE 30, i'm just pointing out that while the 20 will fly it, the weight and drag of the SS will need you to be above 1/2 throttle.

if you do go with the 20 though, the 17x6 is a good starting point.
just figure to add some lead in the nose for balance.