[sz3]

Multiple sizes isn't a bad idea. I'm not sure I know enough about what sizes would be appropriate though. :)

Transfers getting stuck like that usually is an indication that the decoder can't reliably find the 3+1 corner pattern. I might add a toggle to the decoder app that trades speed for more reliability.

That said, there are some failure modes that are harder to fix: I had a mysteriously slow transfer that was driving me nuts, until I noticed that my mouse cursor was on top of one of the corners.