How to | Make the Fonts Bigger in My Notebook
The Wolfram Language allows you to control font sizes of text, math, and graphics for clarity, compactness, or personal preference. You can choose styles for individual characters, whole documents, or application defaults, controlling them according to stylesheets or the output medium.
If you have a number of instances in your notebook where you have to change the font size manually, you can consider the option of creating a stylesheet that defines the font size. Stylesheets in the Wolfram System offer very precise control over various aspects of text formatting.