If the question is about fonts which are used in the task table and other textual content in PDF, then no, it is not configurable at the moment. GanttProject uses 12pt font size. I filed a new issue: Font size option in PDF/PNG export · Issue #2220 · bardsoftware/ganttproject · GitHub
If the question is about the chart image which is exported to PDF, then it depends on the project. GanttProject creates a vector image on a single page. It may appear small e.g. if your project is “tall” (that is, it has many tasks running in a relatively short timeframe). You can export such projects using A0 paper size and then tile the result using third party tools. There is a comprehensive discussion here: