On an iPhone

Open Settings → Display & Brightness → Text Size. Drag the slider until the sample text feels comfortable. For even larger text, open Settings → Accessibility → Display & Text Size → Larger Text.

On Android

Open Settings → Display → Display size and text. Depending on your phone, this may be called Font size or Screen zoom. Move the sliders and check the preview.

Menu names can vary by phone model. Use the search box inside Settings and search for “font size” if you cannot find it.