Find and select your iPhone "Settings" app.
Scroll down and select "Accessibility."
Find and select "Display & Text Size."
To expand your font size options, you must first turn on" Larger Text."
Switching on "Larger Accessibility Sizes" will give you a greater range of font size options from which to choose.
Use the scroll at the bottom of the screen to select the appropriate font size for your iPhone's text.
You should see the font size above change simultaneously with your selection.
Sometimes, when you are busy or out and about and your iPhone starts ringing, you don't want to pick it up, but you know it might be so-and-so with that important call you've been waiting on.