You would have to create a component in Eagle for the Mini-Breakout board. I recommend you to check the SparkFun libraries and check the component they use for the Arduino UNO footprint. It'll help you to know how to create a similar part for the Mini-Breakout board: GitHub - sparkfun/SparkFun-Eagle-Libraries: SparkFun's Public Eagle PCB Footprints using Eagle 6.0+
On the other hand, there are many tutorials about creating components in Eagle. Take a look at the following suggestions:
- How to make a custom library part in Eagle CAD tool
- How to make custom library parts for CadSoft EAGLE
- Making Custom Footprints in EAGLE
- Designing PCBs: SMD Footprints
You should use the .dxf file to get all required dimensions.
There are many dxf viewers.
I use LibreCAD http://librecad.org/cms/home.html
The menu Dimensions in it allows to get exact horizontal and vertical dimensions from any point.