Try commenting out all Serial.println. By doing that, I got the sketch from
Sketch uses 14,350 bytes (100%) of program storage space. Maximum is 14,336 bytes.
to
Sketch uses 12,920 bytes (90%) of program storage space. Maximum is 14,336 bytes.
Troubleshooting without the debug output will be a real pain though.
You can also try the development branch of MySensors. It consumes less flash space than 1.5.
