Hi, and sorry for the delay.
For the 55AX you use the standard TT-46 size Jett-Stream. Nothing special. We do not size them by length, only by application and engine.
I would recommend an 11x6 to start with. You want the engine turning over 14,500 peak ground rpm. In fact the first few flights and break-in with an 11x5 is a good idea.
The combination of 55/Jett-stream/11x6 is just magic. Many aircraft out there flying like that. There are a few old threads in the Extreme Speed forum that can attest
Don't get hung up on the traditional prop sizes.
What aircraft will you be powering?