Hi
we assume you are referring to default view Work Breakdown with Gantt.
Selecting all or a part of your project activities, you may choose from two different ways:
Menu Format > Bigger or Format > Smaller lets you size up or down the selected text in steps.
Menu Format > Show Fonts provides system font selection dialogue to adjust font size and styles to your needs for all selected text.
Additionally you may give it a try, to first select Format > Clear Style, resetting already applied styles before again changing these.
If these steps do neither work out for you, let us know the view and content not being resizable within Merlin Project.