not sure of the exact size...somewhere around 4500 cm^3 (this was a spud gun, the combustion chamber constructed from 3" pvc pipe), probably weighed in at around 10 pounds . this said, he had a fair amount of room to work with(not to mention a far less hostile environment 0.05 seconds doesn't give the plastic time to warm much. we were running computer fans inside the chambers). piezo mics work on a change of state so they would only register an increase/decrease in pressure not an unchanging background pressure.
Rex