Peoject of very smallvoltmeter with ATTiny44A and ADC MCP3421 was released on the hackaday.io.
The board of voltmeter si 20.5 x 10.5 mm and contains all of components, which you need.
In the first link find schematic, PCB and firmware for microcontrollers.
http://eklima.elektroda.eu/wordpress/wp-content/uploads/2015/11/small_voltmeter.zip

Project website: https://hackaday.io/project/8010-small-voltmeter