Looking at your screenshot - this is opensource SoapUI, not ReadyAPI! - right?
I know you can alter both menu/label fonts and editable field fonts in ReadyAPI!, but I'm unsure if you can only alter the editable field font size in opensource SoapUI
Go into the Preferences, select 'Editor Settings' and in the 'Editor Font' section, 'Select Font' button.
Here you can change font, style and size. Perhaps changes here will also be reflected in the menus/labels?