. Shut down JDeveloper and open a Terminal session.
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Go to /Users//.jdeveloper/system12.x.x.x.xx.xxxxxx.xxxx/o.jdeveloper.12.x.x.x.xx.xxxxxx.xxxx (i.e. /Users/Marcelo/.jdeveloper/system12.1.3.0.41.1/o.jdeveloper.12.1.3.1.41.1). Open the file ide.properties (you can use vi, nano, etc). Any text editor will do it.
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Find the line Ide.FontSize.Aqua=10. It should be something similar to: # The default Ide.FontSize for Mac OS X. Ide.FontSize.Aqua=10. Update the IDE's font size replacing the lines above with the following: # The default Ide.FontSize for Mac OS X. Ide.FontSize=10. Save and close the file. Start JDeveloper again and you should see the updated IDE's font size similar to the picture below.