Open iPhone Settings-Display and Brightness, pull to the bottom of the interface, resize the font in the text size, or open bold text:

Use a larger font

If you want to make the display font larger, you can open Settings-Accessibility-Display and Text Size, and then click Bigger Font to set it.

Set fonts in Safari browser

When you view a webpage in Safari browser, you can open the reader view to browse among the supported pages:

Then click "Font" settings, and you can use four fonts: square, song, italics and circles.