I can't find any useful information about the Mega2560 R3 Pro Mini, no datasheet no info no explanation of pinouts, and I don't have a sample to check. This was made entirely from online photos and guess work, so you need to throughly check it before production because I can't verify it's accuracy.
From photos it looks like a compact version of the official Arduino Mega2560, with it's better Atmega16U2 chip, but the DC power supply has been removed. Other photos show it plugs into prefboard, so I assume it's 21 x 15 pins on a 0.100" pitch.
All available info - https://www.google.com.au/search?client=opera&q=Meduino+Mega2560&sourceid=opera&ie=UTF-8&oe=UTF-8
This Meduino Mega2560 Pro Mini board is a small version of the Mega2560 board. It's completely compatible with the original Arduino Mega2560. For the original Mega2560 it has a big board size, some time it maybe not easy to embed it into your system, the Meduino Mega2560 Pro Mini board will be more easy for embedding it into your system. It's cute and small size.
Completely compatible with original Arduino Mega2560
Pin pitch: 0.1 inch
With Atmega16U2 chip as the USB to Serial converter
5V working voltage.
Analog Input Pins: 16
Digital I/O pins: 54
V1.0 - Mega2560 R3 Pro Mini - Mega2560 R3 Pro Mini.fzpz (44.9 KB)
There are versions without headers, but this the version with female headers attached to the underside.