As you know, an XPath expression tends to be a long consecutive string of characters without white spaces. It’s very hard to read in small font.
I looked into
Window > Katalon Studio Preferences > General > Appearance > Color and Fonts
dialog and tried to find which entry to change the font size, but I could not find the entry for Test Object definition pane.
I believe that in macOS we cannot do it via function for font size like in Windows.
Here is the instruction for you to do it through the scaled resolution and its downside is that all the other elements will get bigger too.