Ok, so you know the basic settings for the font size.
Yes, it is possible in IntelliJ to have a shortcut for the font size of the files.
For a shortcut go to
File -> Settings -> Editor -> General
Click the General and in the right panel there will an option Change font size (Zoom) with Ctrl+Mouse Wheel under mouse section.
Enable this setting and apply.
Now you can set the font size of the currently open file by Ctrl+Mouse Wheel and it will not impact the size of other files.
If you want to increase the size for all the files at once then go through the method you have mentioned in the question only i.e. the basic settings for the font size.
Hope this will solve your issue. :)