I guess there are so many ways to customize the look and feel of your app including the font-size.
1. Overriding Variables
You can put your font-size customization to variables.scss that is located inside your your_app_folder/src/theme/. This will affect all the Sass variables found in Ionic’s source scss files.
2. Modifying app.scss
You can put your font-size customization to app.scss that is located inside your your_app_folder/src/app/. This will affect your styles globally for the entire app and not just one component.
3. Modify specific component
You can put your font-size customization to your-component.scss. This will the style of that specific component, you can use this approach if you want to modify a specific component only.
I am not sure if this answers your question but I hope it can help you in any way