Mac Yosemite 10.10 Multimonitor

This applies to Mac OS 10.9, 10.10 and MXWendler version 5.0 and above

Problem

You want to open an output window across multiple monitors but the output window does not want to grow larger than one single external monitor no matter what values are used for output window position and size.

Solution

In 'Mission Control Settings' in 'System Preferences' disable 'Displays have separate Spaces'. This also disables the menu bar in secondary monitors.

Picture linked from osxdaily.com