I have downloaded a Java program with a GUI. I do not have its source.
I am wondering if there is a general Java configuration to change font size in all programs (or, even better, in this one program)
This program seems to be made with Swing
Lot of details are missing here. I couldn't find a link for it online which would have answered many questions. But on a general note, unless application specifically exposes an option inside its configuration file or as a commandline option for font, you cannot set a font size for the entire app. IMO, look inside a user manual that should tell you how to achieve this.
Link above talks about specifying a directory of fonts that will be avaliable to the application. Mind you that link talks about IBM JVM and not Oracle JVM( Formerly Sun JVM)
This link listed all the available system properties for java command and font isn't one of them.
This question should be posted on StackOverFlow for more exposure and more precise answer.