Issues with Screen Scaling on External Monitor
I think changing the screen resolution on the external display could help you in this case. Initially when you open up the Displays Settings from System Preferences. It would look something like this below with a limited set of display resolutions.


This list of resolution is what OS X thinks is the best for your Display. When connecting an external monitor/display, there are chances the aspect ratio is wrong because it's a widescreen monitor. 

Solution


Open the Display Settings from System Preferences on the second monitor. 
Hold the Option key and click on the Scaled button. It should open up a lot more Display resolutions. 



Try finding what's the best option for your display/monitor by trial and error.