This piezo buzzer measures just under 23mm not including the mounting holes. It operates on 3 - 24v and it's quite loud.

Manufacturer : AUDIOWELL
Manufacturer Part Number:  HYD2312
Operating Voltage: 3 - 24V
Rate Voltage: 12V DC
Buzzer Type: Piezoelectric
Frequency: 3900±500Hz
Termination: Lead Wire
Drive Method: Drive Circuit Built-in
Sound Pressure Level: 87/30dB
Max. Current Rating: 10mA
Housing Material: ABS
Operating Temperature: -20C~+60C
Weight: 14g