1.2 What Size?

1. The smaller the transducer diameter, the less efficient the transducer is at creating an ultrasound pulse. In addition, smaller diameters make the transducer less sensitive while trying to detect a pulse. So in general, larger transducers improve signal to noise ratio at the expense of surface area on a container wall and cost.
2. The size of a transducer is also influenced by its material composition.