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